If a game has a chance to crit, chance to miss, sleep etc, it will always be unfair. But also more exciting. This is by design, it works as intended.
If someone is not mature enough to expect those failures and simply can't handle how the RNG system works in games (and how life works in general - you can't always get what you want) then they should just buy a diary and pour their sorrows there, instead of acting like spoiled little princesses, "unfriending" people and being obnoxious in All chat.
Also, none of the currently available legendaries exceed total of 600 base stat, which is shared by many standard pokemon, like for example Metagross, Goodra, Hydreigon, Tyranitar. Do you think those pokemon should be banned from PvP as well?