In your opinion, how tragic can a FCs backstory be before it gets too tragic? Any kinds of backstories that should be avoided altogether?

its not that tragic the very bad thing is that my FC died in a war as a H-Hog

It really depends on the character.

If a character has a tragic back story, they shouldn't whine about it, but enough of the trauma should come through into their personality- Not just how they react to it.

A good back story can be very tragic if the লেখক can pull it off with their character. It আরো so depends on the author's character skills than the character themselves.