From what I gathered from the more-veteran legal folks, a lot of insurance policies (contracts) are written vaguely specifically so that there is room to negotiate as and when a claim is submitted. What that really ends up meaning is that you will inevitably and eventually work with people whose approach to getting what they want is digging in their heels, ignoring the opinions of everyone else, and screaming - literally or by proxy - to get what they want.
Whether you're trying to help them, whether you agree with them or disagree with them, and whether they end up being right or wrong, some people - often those who have been in an industry for dozens of years - will simply not listen to anyone else. It is imperative, especially as a young professional, that you manage your emotions better than they do.
So yeah, I'd say that the worst kind of client is the one who's a few years from retiring, who has enough experience that whatever they say has weight (even if they're uninformed on a topic), and who has no problem being mean.