That was me, and this is really interesting, because I completely disagree with you. I don't think the very real fact that I sometimes make errors means I "can't use the system" or that it needs to change. I think the fact that I sometimes make errors indicates that it can be challenging. I make errors when playing chess, too, and that game is perfect. I don't think it should be dumbed down to the point that I can play it without error.
Having said that, repeating myself again, I do like a lot of what they're doing here. And they can keep the streamlined design while also providing some guidance on playing Vecna as a D&D spellcaster. If I make an occasional mistake, that's fine -- my players do, too, because D&D is strategically and tactically challenging and engaging (at its best). I really don't think we want to replace that, whole cloth, with a series of dice roll exchanges to see whose hit points reach zero first.
But, when you make errors in chess, are you making play errors - as in making a bad move - or rules errors as in moving your knight to the wrong square or moving a rook diagonally? I imagine it's the former, not the latter. So, your example doesn't really say what you think it says. You make bad moves in chess, which is fine. Making a bad move is expected, really, or we'd all be grand masters. But, you don't make mechanical errors, I assume.
Yet, you admit that you cannot keep the rules straight during play. Again, I hardly think you're alone here. Watch any live play and you'll see DM's with gobs and gobs of play experience making mechanical mistakes all the time. It's like I said with the 1-2-1 rule. Sure, that's a fine rule until someone runs or you try to calculate the range to a target that isn't exactly othagonal or diagonal to you. I guarantee that everyone makes mistakes with that system and makes them all the time. Which is why we don't use that system anymore. Any gains in accuracy you might get with 1-2-1 are lost through user error.
The same goes with caster enemies. Running a caster is complicated. Running a high level caster, when you haven't actually played that character, in the middle of combat is so complicated that I would bet dollars to donuts that 90% of DM's using those rules will make mistakes. Most will be minor mistakes, but, they'll make mistakes. We all do. Because I wrote the adventure a couple of weeks ago, then last week's session got cancelled because of real life, so, I haven't looked at the stat block in two weeks and we're three hours into a four hour session when the encounter begins. Of course mistakes are going to be made.
So, what's the point of having these detailed rules that are only making play more difficult and mostly won't even see play at the table? An 18th level caster has what, 25-30 spells known at any given time? At the absolute outside the DM is going to use 10 of them and most likely 3-5. So, most of those rules are just taking up space and will never see actual use at the table.
The whole, "The statblock is the in game description of the monster" really needs to finally be put to bed. That was only ever true in 3e.