I'm sure there are plenty of shoddy insurance companies around. You'll even got poor treatment from bigger names sometimes, so there's no guarantee. I've had very good experiences with State Farm and Geico, but they're not immune from complaints.
I don't know California law, but you might look in to their Deceptive Trade Practices Act (or equivalent), as well as the California Insurance Code. Specifically, look up unfair insurance practices (or something similar) and see if any of your experiences, especially the denial, fit the code language at all. If so, consider calling an attorney specializing in insurance law. Or, if you don't feel comfortable reading statutes, just call an attorney anyway. Many will consult for free. I'm not a big fan of litigation, but really despicable business practices should be addressed.
Source: I'm an attorney (not, of course, in CA).