Honestly, she’s fine in other roles. There was just something about Keiko that didn’t work and I do think it had something to do with her interpretation of Keiko. But she’s been in lots of other shows (she got a lot of notice for her performance in MAS*H) and she did a perfectly good job in them. She would be very unlikely to have 139 acting credits to her name on the IMDB if she was not a talented actor.
So I just can’t say what went wrong there. It wasn’t the writing, because totally different writers were handling her on DS9 than they were on TNG, but the problem existed on both shows (although certainly TNG to a lesser extent). It wasn’t that she isn’t a good actress, because (at least IMO) she is a good actress. So that leaves two options that I can think of, feel free to come up with your own:
1- She interpreted the character all wrong and by the time they really developed her, it was too late.
2- Rick Berman fucked around behind the scenes as usual.