One problem you'll run into on many shared hosting plans is ulimits. It's not just that they'll get upset and cancel your account for using 1 GB of RAM. It's that a single Apache process might not be allowed more than, for example, 128 MB.

As for specifically Yahoo, I wouldn't know anything about hosting with them. I've never used them nor seriously considered using them for hosting. I have had an email account there since about 1994 or 1995, but I've never used them for anything business-related (besides the odd chance that I'd use their IM system to contact a business associate).