Is it just me, or is this a rather weak spell for being 6th level? I mean you create three ghouls, not even ghasts, and being realistic here you're using them as meatbags for the total 66 HP they can tank, since you're certainly not hitting creatures at that level with a +4, and fireball is better at dealing with crowds. But even then, one fireball from a flameskull and they're gone, polymorph is way better at letting someone tank since if you're a T-Rex, you get 136 Temp HP. So, beside obvious world-building reasons, what's the point of the spell?

Quote from Arkzilla2 >>

Is it just me, or is this a rather weak spell for being 6th level? I mean you create three ghouls, not even ghasts, and being realistic here you're using them as meatbags for the total 66 HP they can tank, since you're certainly not hitting creatures at that level with a +4, and fireball is better at dealing with crowds. But even then, one fireball from a flameskull and they're gone, polymorph is way better at letting someone tank since if you're a T-Rex, you get 136 Temp HP. So, beside obvious world-building reasons, what's the point of the spell?

well it's a very strange comparison to make between those two spells that aren't really comparable, but a few notes:

• the 2024 versions of all of these creatures are yet to release, so any one of them may be stronger or weaker
• polymorph has a much greater opportunity cost, as it prevents you from concentrating on other, potentially stronger spells and prevents the beneficiary from using any of their class features (also does not last the entire day and prevents communication, certainly it is a powerful option but using it means not using other stuff)
• polymorphing into a t-rex requires fighting in an area that is capable of housing a t-rex in the first place
• "tanking" in the first place relies on forcing other creatures to focus on attacking the tank, Defenders in 4e had core class mechanics to penalize enemies for attacking other creatures, certain pathfinder classes do, but in 5e such class features are rare indeed, and your enemies do not have much reasion to prioritize the t-rex rather than the caster concentrating on polymorph
• even if you only ever fight creatures of an exactly equal level to your party, three 25% chances to hit per round will score at least a few hits occasionally.
• create undead can be used with any unused 6th or higher spell slots at the end of the day to give you disposable minions the next (and something that can help to keep watch when you long rest)
• back in the 2014 rules, a necromancer's undead thralls feature would make these undead a little bit stronger, and even in the current version each creature created gets to benefit from resourceless or once-per-creature effects like inspiring leader, animal shapes etc. Of course such effects kind of benefit animate dead a lot more.
• the upcasted version that gives you wights is fun since while certainly having a quite big opportunity cost, it is marginally more efficient at getting you zombies per spell slot (up to 24 zombies with an 8th level slot)

the spell is probably still a little bit undertuned, but i feel like you are very much comparing it to the wrong things

Also, polymorph takes an action to cast, while Create Undead has a casting time of one minute. It's not meant to be used in combat.