What are the commonly problems a web host do encounter?

Can you be a bit more specific?

Clients using too much disk space. CPU and memory can be quite a common problem.

One common problem is clients who pay late. Another would be dealing with spam.

Clients take up too much resources that will affect others,so there are some clients will complain.

Network outages and hardware failure are some more problems that may occur.

Fraudulent orders.... Namely letting the bad guys in who try to spam the world or setup http torrents.

I had this happen once and it caused me a world of pain. You get blacklisted, which may cause you fines from your ISP, your get on spam block lists, etc. I know setup things different to handle such things and haven't had a problem since.

Clients use more resources that affects to other clients and another is dealing with spam.

Spam seems the be the biggest issue. If you can find a good mail provider to offload clients too for a minimal cost, i would suggest it

For me, Spam seems to be the biggest issue. Another big issue is harmful script.