Would you say that existentialism is a useful philosophy or a limiting one?

I would have no comment but that wouldn’t make a very good answer now would it? Since you have brought up these interesting yet elephantine concepts that are quite broad I shall attempt to answer in equally broad terms. Your question is unspecific with regards to the nature of its intent. I must infer the meaning from it thus my answer in short, it is both. To elaborate if you mean the collective understandings and ideas that we know as existentialism, then it is a useful and different way of understanding the world. It enriches all human knowledge by adding to the combined wealth of study of the fundamental problems in the area we call philosophy. Furthermore it increases the global knowledge diversity and expands into areas outside of philosophy in a scope that is immeasurable. Even a simple understanding and awareness to its existence allows your views and understands to be challenged and expanded. However if you instead were saying that you should in some way forgo all other understandings in place of this small area of consolidated thoughts known popularly as existentialism. If that case it is most certainly a limiting one.