It is not savegame compatible and there are great threads in the unofficial modding section of this forum to get it all working.

Basically for BI, you will need to do this if you 1.1 and the trivial script installed:

https://forums.totalwar.org/vb/showp...60&postcount=7

And this to get it all working:

https://forums.totalwar.org/vb/showp...3&postcount=85

See this thread for additional info:

https://forums.totalwar.org/vb/showthread.php?t=101745

I have no experience with ALEX, but I play with BI and love it. I've heard that it has more CTD's than regular, but I can't tell because I've been playing with only BI for about a year; though I do get an occasional ctd, but no big. To me naval invasions are the best part, though they can get somewhat annoying, especially when epeiros, KH, and carthage are constantly trying to wrestle sicily and southern italy from me in my romani campaign, though I also like night battles (requires more editing, see the additional info thread).