The animation in the '80s universe is choppy to poke fun at how bad some of the old animation was in episodes. Plus, there will never be an exact match since 2D animation is digitally created now vs. cel-based animation.
That's part of the problem for me. That it was "deliberately" making fun of it. I know Forever did it, but that was in the acting department (not to mention the fact that they really did start getting that goofy later on). When it came to animation it was on par with, and even better a lot of times, than the "Neutrinos" episode of the original cartoon. They moved and animated like the Turtles. The ones in this special do not, and it kinda throws me off. I'd rather there not be jabs at all towards the original in many cases, but if there are going to be I prefer it to be something about how goofy it got story/character wise rather than something that isn't that pleasant to look at.
As for the inconsitencies in Krang's backstory, you don't have to take it canonical to the old series (which was somewhat loose with continuitt anyway) but just accept it for this episode.
Like I said, it's obviously just an excuse plot for the episode, and it wasn't that intrusive. BUTTTT my mind still kept trying to tap me on the shoulder with how much the episode was screwing up. It was just annoying enough inconsistencies to make it start get annoying for me, especially after how amazingly Forever handled the multiverse situation.
The reason for the "no Shredder" should be fairly obvious, but in case it isn't, let me spell it out. You can't have a "nostaglia" Shredder in an episode when the voice actor who played him for a large part of the 80's series is no longer with us.
That's obviously the explanation, but even then, Krang just doesn't work that well, ESPECIALLY in a scenario where they're kinda messing up what he is, without his partner for me. At the least you could get the Forever voice actor.
Finally, it's fairly clear that "Turtles Forever" didn't happen in this version of TMNT. Think of this episode as a replacement for that movie.
That sounds like complete and utter blasphemy to me.
Now I'm fully behind the "alternate 80s cartoon" idea, but this episode is clearly trying to work off the 80s cartoon itself, which just doesn't jell with me. Hence my criticism.
It's clearly all just an excuse for a crossover is all. Makes no actual sense, given how Krang is in the original cartoon, that he'd be from the same Dimension X that the Turtles of the new cartoon are having to deal with.
My thoughts on the crossover are what I thought it'd be. A little get-together that'd be a wasted opportunity. Yeah, it was nice having the original voices, but Donatello's really doesn't fit with how it sounds in this. It is nice to see the different art styles. Unfortunately the animation in the original cartoon's universe is REALLY choppy and Krang...kinda doesn't look right. It looks like a parody of the original cartoon rather than the original cartoon. And that's not even getting into the story problems. I know it's just fanservice is all, but with how it's trying to act like the whole dimension-thingy is being sorta consistent from Turtles Forever it just falls flat on its face. Even if Krang was based off the Utroms, he is NOT one in the original. He LOST his body when he was banished. And he was a tyrant. And his Dimension X was COMPLETELY different from the new cartoon's one. I know I'm looking too far into it, but it's just basic stuff that pops into my head as I'm watching it that kinda brings the thing down. Also no Shredder.
...and shouldn't at least ONE of the two sets of Turtles remember each other between the Mirage and original cartoon Turtles? On the other hand, if this is their first time meeting then they should remember each other in Turtles Forever.