Agreed. Now that Blade of Disaster is out in Icewind Dale, Mordenkainen's Sword really needs an errata...

If only Mordenkainen's Sword (a) wasn't concentration and (b) did two attacks as an action (rather than one attack as a bonus action)

Then we could be a sword saint with two magic swords swinging for 4 attacks a turn 😍

2 attacks with Mordenkainen's (action) + 2 attacks with Disaster (bonus action)

Plus maybe some Tiny Servant daggers? 

But Spiritual Weapon requires no concentration, so you could use it in conjunction with other spells like wall of fire or Bigby’s Hand (if you have acces to these spells). And Spiritual Wepon adds your modifier.

For those saying this weapon is worse than spiritual weapon, just some things to keep in mind:

• Cleric spells are designed to be very powerful: the average damage of spiritual weapon at 8th level with a +5 modifier is 23; Mordenkainen’s Sword would have an average damage of 33 if you hit twice on your first turn you summon it, and an average of 16 on subsequent hits. In 1 minute of constant hits (highly unlikely for both spells) spiritual weapon would output an average of 230 damage and Mordenkainen’s Sword would output an average of 181 damage. The design is to give wizards a form of spectral weaponry they can summon without stepping on the cleric's shoes.
  
  
• You wouldn't cast spiritual weapon at 8th level considering you only get one 8th level spell slot; you'd be throwing away any powerful 8th level spells that could change the dynamic of the battle for a little more damage output. Remember that damage isn't everything, especially at high level when conditions and status effects are the difference between life and death.
  
  
• Arcane Hand can be destroyed by damage, where as Mordenkainen’s Sword can't.

If I were to improve this spell (which yes you could improve it in ways to give it some better play for wizards while keep it fair for the cleric the poor church boyz):

1. I'd change the damage from 3d10 to 3d12.
   
   
2. I'd change the speed from 20 feet to 30 feet.

This just reads like a worse spiritual weapon. I'd probably take that cast at 2nd level over this spell. Average damage difference will be negligible and it won't need concentration.

Keep in mind you can use it twice on the first turn, as part of your action casting the spell, and then as a bonus action.

It deals d10s, not 12s. Even if it did deal d12s, both Bard and Wizard have a slew of other spells that outdo this by miles

Let's be real, this should be a 4th level spell. The concentration requirement means that it should be able to outpace spiritual weapon damage scaling, and the bonus action attack makes it better than call lightning, which does the same damage and can be used on subsequent turns, but takes an action and affects a 5 ft sphere. 

This spell is really underpowered.  My adventuring party encountered FOUR Mordenkainen's Swords in Maddgoth's Castle from Waterdeep:  Dungeon of The Mad Mage.  We were all at level 9, where our highest level spell slot is 5.  Since this is a 7th-level spell, theoretically, this encounter should have killed at least two of our characters.  But we actually survived, which shows that this spell is literally dogwater.  After we disengaged from the room, I healed myself and the rest of the party up with a healing spirit, and this is when I discovered firsthand that healing spirit is actually broken, the article was right.  

The swords got lucky with the damage rolls, because all of the attacks were above 20.  But the scary part was that there were FOUR of them and we didn't know until the DM revealed it outside of combat.  We tried to attack them a couple of times, thinking that they were just flying swords, but we soon realized that the swords weren't creatures or objects. 

The only purpose of this spell is the fact that wizards and bards can't cast spiritual weapon.  Spiritual weapon requires no concentration and deals (6d8 + spellcasting ability mod) force damage.  But why learn this when you can spend your precious gold, time, or known spells on a more potent 7th-level spell like prismatic spray, plane shift, or teleport.         

It might be true that you can attack twice with this spell on the first turn but that's not really a benefit. 
You can use spiritual weapon and an action including a cantrip together.
This spell, 6d10 using both your action and bonus on your first turn, then 3d10 every turn bonus action. Can move 20 feet.
And 7th level bigby's hand? 8d8 using just your action on your first turn, then 8d8 every turn bonus action, has other options, can move 60 ft.
Cast at 8th level and it's 10d8. I know bigby's hand can be destroyed but it's still not even a competition here. 
This is just a blatantly bad balance. It's bigby's hand's problem just as much as it is this spell's, because it probably shouldn't add 2d8 force damage per upcast spell level to a bonus action attack. The only thing this spell does is let bards use it without using arcane secrets, other than that it's completely replaced by bigby's.

Quote from EnchantedNinja12345 >>

I healed myself and the rest of the party up with a healing spirit, and this is when I discovered firsthand that healing spirit is actually broken, the article was right.    

Healing Spirit got nerfed, it only heals spellcasting mod+1 times. It's honestly very weak for an out of combat heal now, but it is versatile and can all be dumped on one person if need be.

Quote from ArtemisCaresTooMuch >>

Spiritual weapon level 6: 4d8, averaging 18

This Spell level 7: 3d10, averaging 16

 

 AND spiritual weapon does not require concentration.  OMG!  Wait.  Mordenkainen's Sword can only move 20 feet per turn, while spiritual weapon can move 60.  

Here's my rework on Mordenkainen's Sword, this should make it more playable:  https://www.dndbeyond.com/spells/1381547-mordenkainens-sword-rework

if you pick this over crown of stars, seek immediate psychiatric help

tl;dr: if there's enough room and you can see it, prefer arcane/bigby's hand over this spell, because rare should be the times you can't see and are able to safely utilize a familiar's blindsight to move and attack with mordenkainen's sword, maybe you're too fond of convoluted combos, maybe you should look at different spells

mordenkainen's sword is a combo spell of at least 7th level for the purpose of counterspell, dispel magic, spell scroll, etc, dungeon master probably doesn't have to worry about it being an overpowered spell scroll reward

It is the only spell in which the Bard gets to roll an attack die, instead of the DM rolling a save.  My bard complains about not getting to roll dice on their turn. ;P

Not everyone is a cleric multiclasser. Spiritual Weapon is only on the Cleric Spell List, while Mordenkainen's Sword is on the Bard and Wizard Spell List. Seems to me most people looking up this spell would likely be a Bard or Wizard...

Of course inflict wounds will deal more damage per turn. It's a one turn spell. A spell you can use 5+ turns in a row isn't going to deal the same damage per turn as a one off spell of the same level.

Remember that arcane hand can be destroyed. Mordekainen's sword not. And multiclassing is an optional rule (so can't be used in discussion about game balance), so not everyone will use it. This spell can have some use for wizards (who don't have many opportunities to use bonus action). And maybe some bards (who don't want to waste magical secrets for spiritual weapon). So maybe it's not a top tier spell, but also not utterly trash.

To make this spell not utter garbage for a 7th level, I rule in my campaigns that it's invisible and can affect creatures on the ethereal plane, similar to other force spells like wall of force and forcecage.

It shouldn't take concentration, or a bonus action to use on subsequent turns. Currently a waste of a 7th level slot