I actually like a lot of nns and there's quite a few that I almost prefer to the name!
But here's a few names that have stumped me as far as nns to use:
Francesca - wouldn't use
Fran /
Frannie or Cesca, and definitely not
FannyVictoria - wouldn't use
Vicki and
Tori is cute but not solid enough
Sebastian - Seb? No thanks. Sebby? Weird.
Bastian I guess could work though.