That game was awesome.
I beat it on the Genesis, after weeks upon weeks of trial and error.
The last park in the game was really hard, in which all the villains come to attack you, including shriek, demogobling, etc.
Then the last stage in which to you could switch between Spiderman and Venom at will. Carnage was extremely hard on that one last level.
Regarding Separation Anxiety, i never heard about it. Then after years of having a PS i learned about the existence of that game, but never actually played it.