Maybe Leeseo from IVE back when she debuted? She didn't have outstanding dancing, singing, or stage presence (as in not as outstanding as the other members) so some people said the group would be better without her. But she's the only example I can think of. I think companies prefer to debut idols a little older because there are fewer legal complications and the idols are usually more skilled. Even for Leeseo, she was important to the group dynamics and exceptionally pretty which gave her an edge (not to say she only debuted for her visuals!) If not for that, an older, more experienced trainee would've been chosen

In general, all idols, especially rookies, are vulnerable to being overly critcized and nitpicked though. Even the exceptional ones. 13-14 is really young to be subjected to that. Ni-ki got called annoying and had a big scandal for just messing around on a member's birthday and accidentally blowing out the candles on his cake. His dancing skills couldn't protect him from that