... Are there any functions in BIAB not available in RealBand?
Yes.
Depending on how you work you may notice them or you may not.
BiaB is much faster at generating/creating new ideas; it is setup to try a lot of different variations/styles very fast. RB is more of a "I want to do this here and that there' type setup. It requires more input from the user, but also offers some features/controls that BiaB does not, so your question can go both ways.
If you have some DAW experience and familiarity, you may find the RB workflow much more flexible and powerful. It's more linear (like most other DAWs). If you don't, BiaB may be more suitable to you. There are many users who don't seem to mind the difference between the two. They are separate programs for a reason. I hope this remains so for the foreseeable future. Both have their strengths.
They are very different in how they actually perform, but both do indeed share a lot of the same functions .. it's all about your workflow.