I don't have Strong Opinions about starclan, but I've been thinking about this a lot
and having a lot of trouble coming to a real conclusion so I hope this makes sense :P
my initial thought was similar to what dfw said about the questions their powers raise, because I'm not sure how consistent they are, or at least come off as? which I kind of understand, given how the story has evolved - they started out as more belief than fact, but have appeared more and more until we could confirm that they're real (eliminating the mystery), and also not all-powerful (eliminating the awe). so for people who enjoyed them as a vague mysterious belief system, that change alone isn't appealing
but more than that, it might have left starclan in this middle ground where they sometimes can/do and sometimes can't/won't help the living cats, and without set rules about what they can/are allowed to do, it's easy to feel like they're being used just to fix things or screw stuff up, which of course comes off as cheap. uncertainty about how much power they really have can easily lead to dissatisfaction with what they do - if they CAN help but don't that's cruel, if they can't help but won't go away that's obnoxious, etc.
so I guess what I'm saying is, the authors may have been better off picking a direction and sticking with it - making starclan relevant characters governed by clear rules, or mysterious ancestors who don't get very involved, but not both.
either way, I do lean toward them having some power over no power, because the whole point of starclan is that they're supposed to guide the living cats, and if done right that's fine. the religion/faith of the clans is that they believe their ancestors are watching over them and sending them signs to help them, so if starclan didn't know anything they'd be functionally useless. which, as has been said, would make for an interesting story, having the clans' beliefs turn out to be completely untrue, but probably one beyond the scope of what the authors are going for here
but also, it's occurring to me now that starclan as we usually see it isn't necessarily one unified body? it's made up of lots of cats who may have different personalities and experiences and ideas about what's "right," and iirc when "starclan" is involved it's mostly just a few characters speaking for them, which makes me wonder if the bigger problem isn't what starclan can or can't do, but simply these starclan cats leading the living cats around. prophecies are one thing, but dead characters getting involved for personal reasons can quickly start to feel less like guidance and more like interference. so regardless of how powerful starclan is, maybe it's really an issue with life and the afterlife not being separate enough, and the dead not moving on?