I'd mention the various Magikarp fishermen throughout the games, but you did say something about build up soooo...
Anyway, DP flint was a pretty big dissapointment primarily because he has only two fire types. He makes up for it somewhat with Sunny Day I guess and double team BP Drifblim is just asshole-ish.
Janine, as already mentioned, is hilariously weak, more so than probably any other "boss" trainer in the game. In GSC, her Pokemon were around 10 levels lower than the surrounding gyms and probably 10-20 below your own, on top of just being generally shitty to begin with. Though the levels get upped in HGSS, her Pokemon are still lower in level than the other Kanto gymleaders and they're just as weak. When your best Pokemon is a VENOMOTH pre-gen V that's seriously sad.
Another one worth mentioning is Brycen, who is just saddled with bad Pokemon with bad typing. If you have any Pokemon that have a type advantage over him you're set, and considering his type is Ice you have to be TRYING to not have a type advantage. Worse, they're all pure Ice meaning they don't have decent secondary types to fall back on like Pryce did.