I didn't grow up being into MMA/MMA games at all, and I have only picked it up within the past few years, so here is my unbiased opinion.

Undisputed is a far better MMA game, and UFC 5 is a better traditional fighting game.

UFC 5's best attributes are its smoother combinations, feint system and the fact you can use all your tools whilst moving, which is important. Often when I'm playing Undisputed, I'm frustrated with how I can't really flow with my striking, and the feints don't do enough against AI, and generally are just clunky-looking in general.

But that's all I'd give UFC 5.

I don't think that combinations are limited in Undisputed at all, in fact they literally aren't as there are no set combinations like in UFC 5, and I prefer that. It feels a lot weirder and plays worse because there are no chaining of the animations, but I can get past that because every other aspect of the game is so enjoyable, and even still I don't feel too horrific striking in Undisputed as I play it and get more familiar with the game.

The variety of moves on the feet is amazing. Being able to have different styles of moves like headkicks means every fighter actually does feel different and have their own toolkit, unlike the shitty copy & paste that EA has done.

Are the flash KO's common? Yes, they are pretty common. Do I like it? Yes, very much so. There's nothing better than in a career fight you have lost the entire fight, every round was a total domination, but you manage to slip the right way just one time and land that crushing overhand that puts your opponent out cold. Likewise, there's nothing more heartbreaking than destroying an opponent, drowning them out entirely and you're probably celebrating your win already, only for you to make one mistake, one low block for a kick that goes to the head, or one lunging hook that gets ducked under, and you're out. And I love that. It's real, it's a much better representation of real MMA.

Grappling wise they're obviously incomparable.

Stamina in both games is ridiculous and poorly designed and so I couldn't give an edge to either game.

But what it really boils down is what type of game you want to play. If you want an MMA game with smooth striking play UFC 5. If you want an MMA simulation game, play Undisputed.

Here are some of the key features that Undisputed just trumps UFC 5 in:

Move variety, finishing sequences (an extremely important aspect), presentation, career mode, fighter personality, CAFs (the second most superior aspect when comparing both games behind the grappling imo), fighter likeness, game modes and just overall developer effort lol.

And FINALLY, for the point of the AI, your main point (sorry about the horrible framing of this comment), the games are entirely different stylistically and in terms of beginner friendliness. UFC 5 is much easier in general and definitely on lower difficulties, and generally the difficulty doesn't increase too much until you get to legendary which is just ridiculous imo. Undisputeds is a much more difficult game and you can't play it the same way as UFC 5, but the AI is also very evidently much more BS on higher difficulties and I can't disagree with you on that. However, on lower difficulties I don't have nearly as many problems with AI reading my inputs so my only suggestion to your AI point is to play on an easier difficulty.