There is something to be said for that, otherwise why would SO MANY PCs take it???
In 10 years (ish) of 5e, I can't say I've ever seen a single PC actually take Magic Missile. We've had a few wands of magic missile floating around, which have seen use, but, actually burning a spell slot for it? Nope.
Again, it would be unbelievably helpful if folks would simply talk about themselves instead of trying to pretend that their preferences are so universal that every should automatically just nod in agreement.
Having different abilities is fine. The new Vecna has a lot of features PCs can't have, which is fine.
What isn't fine IMO is when you have a system PCs use, such as a spellcasting, and monsters do it differently.
A Knight has features, but isn't a "Fighter", it is something different. Vecna's statblock actually says "(wizard)"
Wow. Really? That's the defense? So, what, exactly, is a knight? Ranger? Nope, no spells. Can't be a Paladin either. Oh, oh, I know, he's a ferret. Yup, a really, really tall ferret. In armor.
And people wonder why I scoff at the whole simulationist argument. If simulationist play was actually a thing, then the idea that a knight isn't a fighter would blow people's minds. But, of course, it doesn't. It's all about simulation when it's convenient. It's something "lots of people do" when it's convenient to plant a flag.
Again, since no one answered me the first time around, how is it good game design to have a system that is so complex than no one can use it without making mistakes? When even seasoned, veteran DM's cannot use these creatures without making mistakes, how is this possibly a good design? And, it's not like making mistakes with casters is some isolated thing that comes up once in a blue moon. It happens all the time. Because using a caster, particuarly anything in double digit levels, is insanely complicated, and far, far too much for a DM to handle in the middle of an encounter. So, the game grinds to a halt as yet again one of the players corrects the DM, leading to the DM getting more and more frustrated, and generally resulting in terrible encounters.