I found her believable, she was just rather shrill. She never grabbed me the first run through but I've enjoyed her more in subsequent rewatches. Her relationship with Sisko was excellent.. and you're right, Ro and Sisko might have had some problems BUT you would also have an atheist Bajoran whose commanding officer is viewed as a deity by her planet's religion. So there might have been some big time conflict there, or at the very least a whole lot of Ro sarcasm.
Wasn't Ro only established as an atheist in the novels? Given the spiritual theme of DS9 if Ro was casted I bet that never would've happened.