What do you mean by Critical Mode?
A new level of difficulty in Kingdom Hearts 2: Final Mix. It's like a remake of KH2 with extra stuff, released only in Japan. In Critical Mode, your HP bar is cut in half and you have the option of going through the entire game at level 1.

I never found the other ones to be disadvantageous against any enemy or boss
There are LOTS of enemies that have a nasty tendency to break out of your combo in Master and then hit you in the face. I have just tested it out against Demyx, and he is one of them.

Master allowed me to blocked all of his lasers
Huh? Master doesn't have Guard, and his white lasers CAN'T be blocked except by Reflect (and Warp Snipe RC for a certain attack), which Normal Sora can do as well.

Also, Xigbar likes breaking out of Master combos as well, except that he SHOOTS you in the face.

that can't be blocked like that with regular Sora when he traps you on that platform
You mean the one where his lasers can be avoided by literally running around in a circle?
and for anti in that battle I was also able to dodge those same lasers and wasn't be able to get hit until the form wore off as the form made it easier for me to dodge all of his attacks.
Yeah, I know that. But again, make one mistake while in Anti, and you're pretty much guaranteed a game over. This is true for pretty much all modes.