First of all, I'd like to mention that I myself don't get good rates for drops and yes I have opened hundreds of chests with basic TMs and tokens inside only.
The PvP chest rates are completely RNG-based, meaning that whether you get something or not is random. If that's the case, we should consider hunting and everything that requires good effort and time, which can ultimately be disappointing due to RNG.
I also want to mention that if a rare PvP chest item becomes guaranteed at some point, it will decrease the worth of those items, especially cosmetics and mounts.
So for that's it's a -1 suggestion
But something that I'd like to point out is that farming PvP chests isn't that easy, not for those who aren't as good in PvP or just don't know much about new Pokémon. As for hunting, a player doesn't require good skills or need to do much about it. You just get your FS and sync and other required stuff and spam right and left.
Which gives the suggestion a somewhat +1
I personally think that instead of actually guaranteeing a rare item from PvP chests, it makes the overall odds better. Adding a bar is somewhat meaningless when you can just make the odds better. After all, the purpose is to encourage the players and make it a little fair. I actually want it to happen (+1) because I don't get good things. I've opened hundreds and only got the Ulquiorra Set and IV Reducing twice. Still, I'd like it if they made the rates better instead of adding a luck bar.