I personally feel like there are varying degrees of "can't". For example, I can't lift up a bus. That is a simple fact and I physically can't do it. But I wouldn't go as far as to say that I can't be successful at something that is reasonably feasible. Saying you can't do something is usually an easy way out and is used where "won't" or "don't want to" should really be used.
I hope this answer makes sense and isn't too confusing. I don't see the word as taboo at all, and as reeltaulk said, sometime a simple "I don't know" is much better than giving false information.