Ok, maybe I’m exaggerating on the “every episode” but you’re still going to need at least three actors to play the main character, plus intermediate appearances as they transform which lasts an episode or more. It’s definitely a significant additional problem that you just don’t have to deal with if you start with one of the other books.
The other challenge if you are thinking about doing multiple books and you start with Phlebas is that the Culture are the baddies. You spend a (most) of a series presenting them as such and then flip.