In the 2024 rules, at 10th level, monks get a third attack with Flurry of Blows, and they don't need to take the Attack action to use FoB. I was thinking that you could use this multiclass to be able to cast a spell as your action and then still use FoB for good damage. With Uncanny Metabolism, a Monk can theoretically get 40 Focus Points over the course of the day, which, assuming 7 encounters, allows spending about six per encounter, so the multiclass can use FoB every turn, and use Stunning Strike at least some turns.

Edit: the third attack is at 10th level, so you'd do 10/10.

Would a Monk 11/Spellcaster 9 be Really Strong?

Compared to what?   Another 20th level character, then no it would not.  Compared to a 5 level character then yes.

Quote from Maruntoryx >>

In the 2024 rules, at 10th level, monks get a third attack with Flurry of Blows, and they don't need to take the Attack action to use FoB. I was thinking that you could use this multiclass to be able to cast a spell as your action and then still use FoB for good damage. With Uncanny Metabolism, a Monk can theoretically get 40 Focus Points over the course of the day, which, assuming 7 encounters, allows spending about six per encounter, so the multiclass can use FoB every turn, and use Stunning Strike at least some turns.

Edit: the third attack is at 10th level, so you'd do 10/10.

 The ability score would be a challenge (except cleric or Druid) and you would suffer with Con as well. Unless you are rolling your scores and get insane results. 
then the answer is yes. But if not, no chance because you’re weakening your self with other needs.

Quote from Cyrene66219 >>

Quote from Maruntoryx >>

In the 2024 rules, at 10th level, monks get a third attack with Flurry of Blows, and they don't need to take the Attack action to use FoB. I was thinking that you could use this multiclass to be able to cast a spell as your action and then still use FoB for good damage. With Uncanny Metabolism, a Monk can theoretically get 40 Focus Points over the course of the day, which, assuming 7 encounters, allows spending about six per encounter, so the multiclass can use FoB every turn, and use Stunning Strike at least some turns.

Edit: the third attack is at 10th level, so you'd do 10/10.

 The ability score would be a challenge (except cleric or Druid) and you would suffer with Con as well. Unless you are rolling your scores and get insane results. 
then the answer is yes. But if not, no chance because you’re weakening your self with other needs.

 I recently came up with an overpowered monk-druid MC, but I haven't posted it on ddb

Quote from Cyrene66219 >>

Quote from Maruntoryx >>

In the 2024 rules, at 10th level, monks get a third attack with Flurry of Blows, and they don't need to take the Attack action to use FoB. I was thinking that you could use this multiclass to be able to cast a spell as your action and then still use FoB for good damage. With Uncanny Metabolism, a Monk can theoretically get 40 Focus Points over the course of the day, which, assuming 7 encounters, allows spending about six per encounter, so the multiclass can use FoB every turn, and use Stunning Strike at least some turns.

Edit: the third attack is at 10th level, so you'd do 10/10.

 The ability score would be a challenge (except cleric or Druid) and you would suffer with Con as well. Unless you are rolling your scores and get insane results. 
then the answer is yes. But if not, no chance because you’re weakening your self with other needs.

 So very good barring the MAD issue?

A Monk 11/Druid 9 can be very powerful at high levels. As a Monk, you will be making 5 unarmed strikes with Flurry of Blows. As a Druid 9, you would be able to cast Conjure Minor Elementals as 5th level spell to add an extra 15d8 damage per round (3d8 per attack for 5 attacks, assuming they all hit). It's a pretty powerful build, but you will suffer somewhat during the levels between taking Druid 1 and getting to Druid 7 for CME.

I'm not convinced.

The MAD issue can easily be solved by choosing to be a cleric or druid but I am not convinced a Monk 10 Cleric/Druid 10 would be better than a single class level 20 character. 

The damage output of a monk does not increase a huge amount between level 11 and 20 except for their epic boon, (assuming they go for combat prowess), as a multiclass you can still get this boon but youwould not then get a boon that improves your spellcasting (clerics and druids have few if any spells that make attack rolls and even for wizards the best spells require saving rolls).  However the survivability of a monk improves a lot as they go for higher levels with features giving them proficiency in all saving throws, resistance to all damage excet force and the ability to deflect attacks of any damage type, spells like shield of faith and absorb elements can help a bit but you are likely to be using a lot of your actions and spell slots to heal yourself and even then 

Compared to a level 20 cleric or druid you will have nothing higher than 5 level spell slots spells like mass heal, foresight and heroes' feast can enable the party to survive a tough day. 

Really you need to look at it level by level because rarely does anyone play a handful of sessions at level 20. Most of the game is played before then. So starting at Level 11:

Monk: Subclass feature one of (OH) free Step of the Wind as part of your Bonus action every turn (Mercy) Free Hand of Harm as part of Flurry of Blows (Shadow) free unarmed strike when you BA teleport (Elements) you can fly.

Cleric-1 : Add your Wis to Arcana/Religion, 4 cantrips + 2x 1st level slots. Which 1st level spells / cantrips are worth giving up your level 11 Monk subclass feature? Your best options are probably Bless, Guidance, Toll the Dead. Sure you could get a ranged option, but if you were an Elements Monk you could just fly & punch the baddies in the face instead... Bless is pretty good IF you can keep concentration on it, and no one else in your party has it. Protection from Evil/Good can be good in the right situation, or Bane would slightly increase you chance to stun an enemy. 

Druid-1 : Add your Wis to Arcana/Nature, 3 cantrips + 2x 1st level slots. Which 1st level spells / cantrips are worth it? Shillelagh ... doesn't really help, Magic Stone gives you a very meh? ranged attack (TBH a shortbow is better). Guidance, Absorb Elements can help mitigate AoE damage you take but you've already got a really good reaction, Faerie Fire's ok but Stunning your enemy is way better, and I guess Protection from Evil/Good again.

At Level 12: you're giving up an ASI/Feat so falling behind in terms of damage, AC or DC for your monk abilities. In exchange for Turn undead or Wild shape both of which are very niche, or some weak healing / damage.

Level 13: you're giving up Deflect Energy which is pretty useful at these levels TBH, but you do get your Cleric or Druid subclass, your best choice is probably Trickery Cleric for free(ish) Advantage from your Illusory Duplicate unfortunately it eats your BA to move it so honestly not that good, Light Domain would give you a very weak AoE Channel Divinity and a reaction that is just worse than your Deflect Attack. For Druid you've got the Sea option but the Sea WS uses your BA so probably not good unless you are completely surrounded, or Stars for Dragon Form to help keep your concentration. Spells wise you've basically got Aid, Moonbeam and Enlarge/Reduce that are at all helpful to you.

Level 14: you're giving up proficiency in EVERY SAVING THROW - i.e. one of the best abilities in the whole game. For your delayed ASI.

Maybe at Level 15 when you finally get 3rd level spells and can get Spirit Guardians or Conjure Animals for some extra free damage each round, it finally starts to be worth it? I dunno it still seems like a pretty bad trade to me.

That depends,

As you say most campaigns end well before level 20, usually by level 11 most campaigns are coming to an end for those you don't need to worry about level 20.

The vast majority of Level 20 play is in one shots and for that you don't need to worry about levels 1-19.

You’re also giving up a lot of Ki, which could pinch hard if the DM is throwing several high intensity encounters at you in a day.

Quote from Jegpeg >>

I'm not convinced.

The MAD issue can easily be solved by choosing to be a cleric or druid but I am not convinced a Monk 10 Cleric/Druid 10 would be better than a single class level 20 character. 

The damage output of a monk does not increase a huge amount between level 11 and 20 except for their epic boon, (assuming they go for combat prowess), as a multiclass you can still get this boon but youwould not then get a boon that improves your spellcasting (clerics and druids have few if any spells that make attack rolls and even for wizards the best spells require saving rolls).  However the survivability of a monk improves a lot as they go for higher levels with features giving them proficiency in all saving throws, resistance to all damage excet force and the ability to deflect attacks of any damage type, spells like shield of faith and absorb elements can help a bit but you are likely to be using a lot of your actions and spell slots to heal yourself and even then 

Compared to a level 20 cleric or druid you will have nothing higher than 5 level spell slots spells like mass heal, foresight and heroes' feast can enable the party to survive a tough day. 

 I did say besides Druid or Cleric. I was talking about Intel charisma characters. 

oh if you want to include 1/2 caster Monk/Ranger combo works😛

At level 20 you are missing out on too many benefits to have such an even split. A level or three could have some advantages but anything more becomes a big penalty by 20.

Quote from Wysperra >>

At level 20 you are missing out on too many benefits to have such an even split. A level or three could have some advantages but anything more becomes a big penalty by 20.

 I was thinking the same thing. It sounds like a really terrible monk that can do some kind of neat spellcasting, or a really terrible spellcaster that can do some neat punching. 

Imo 50/50 in general is not a good multiclass split- you quickly end up too far behind on either side to really shine.