I am not quite sure how all the melee mechanics are working (or should work) atm. Sneak attack has nothing to do with attacking from behind, right? Because afaik this gives you advantage on the roll, for all melee attacks, not sure what exactly this means in the game though. Highest of 2 rolls?
But then there is the backstab mechanic for the rogue, also not sure how it works.

In general I think there should be better visual combat indicators for the stuff that is going on, similar to the text that pops up for criticals; the same should apply for reactions, backstabs, sneak attacks etc.