I've been told by an OT that they shouldn't be in there for long periods before they can sit on their own as it stops them from developing a sense of balance.
Having said that, I have friends who had a bumbo for their babies because they had them close together and needed it for logistic reasons, LOL. The two kids who were in there had no problems sitting and walking...
They're probably like anything - great if you don't over-use them.