I think the main difference is that with the Euphoria engine the tackles just work themselves out... I think they use rag doll type physics where the game world just knows what should happen to a player if he is hit here... or if he is hit there, with this velocity etc...etc... which means that no tackle will be the same.
The Madden engine I believe is limited in teh sense that they have to actually put all of those animations in... so there is not an infinite number of animations you can have... whatever you motion captured thats what you have to work with....
At least that is how I understand it... someone with more knowledge can correct me if I'm wrong.
Either way, I have to assume that Ian would love a Euphoria engine to build Madden on