I didn't notice any "significant" loss in frame rate, at least compared to several other mods. It's "stable", but there are a few potential conflicts with other mods that change the landscape in the area or make other chnages to Balmora.
Personally, I found it "interesting", but of "mixed" usefulness and lore-compatibility. If you don't mind the stuff that's blatantly spoiler-ish being offered for sale all around you, and things which totally change the "feel" of the game, then it's pretty good over-all. Since it's a compilation of the works of several different modders, the quality, compatibility, and credibility varies by quite a bit from one shop or NPC to another. Ultimately, it turned out that I did not want certain parts of it in my game, and I unchecked the mod, but most of it is great work. I'm still considering adding one or two of those individual mods seperately one of these days, if I can find them.
It doesn't hurt to try it, and there's enough "good stuff" there to at least warrant a look.
My opinion is pretty much the same: you'll have to try it and see by yourself if Balmora Expansion is your thing.
Overall it's a good mod, but being essentially a "merge" of different mods, the quality vary and it stretches lore in some places.
Also, it should be noted that it's pretty old: the average quality of mods has risen a lot. Not only that, but some of the shops look pretty outdated by now.
The "undeground" part especially bothered me... let's just say that difficulty of that section is on the level of Tribunal, so definitely excessive for a starting character; which may come as an unpleasant surprise, since Balmora is the typical starting city for new PCs.
I played with BE for a time, and also modded it a bit to make it better meet my personal taste... in the end, thought, I felt too many things in it clashed with the feeling I like in my games, so I removed it.