Pretty much everything that has already been said, now while I'm not against tragic backstories, they have to have some sort of meaning behind it, for example, the OC was an orphan because their parents died due to illness, a cart/car accident or they were captured and killed during either the Changeling Invasion or King Sombra's reign, and not something like just like "They were abandoned just because" or "The parents were alcoholics/junkies"..
I'm not too fond of alicorns, but if the alicorn say has some form of limitations, like they are an alicorn but they have magical limitations or can't do magic due to an accident or something, that's different, because they are not OP af.
and I think the whole "lost sibling/relative" or "bf/gf" of the mane six or Princesses speaks for it self, I'm not bothered by it if say the backstory is the oc was an orphan and Luna brought her/him to her orphanage/colony (Depends which storyline you follow) because that is neither lost relative or bf/gf of the mane six or the princesses.
For example, I have two ocs (brother and sister) who lost their parents to murder when they were still foals and they run away, then are found by Luna and brought to her orphanage and are cared for by her, thus they look up to her as a motherly figure/rolemodel.