People with less than 200k HP being lumped in with people with 550k HP, Russian premades, screwed up scaling with party sync…
Don’t forget to add some interesting multiboxer sh*** that has started happening recently in random epic BGs (some dude (or dudes) with a whole raid of DKs and Shammans.
I’ve seen them kill Horde boss in Ashran in 25 seconds, despite the boss being healed by 6 healers (not the best geared ones, but still not bad either - who in normal game would have no trouble healing that boss / or any players).
But yeah the gear disparity is annoying, every time I join an Ashran bg , we end up having some tanks with 300kHP, while the tanks in opposing team have 1.5 - 3 mln HP (with buffs) 
Or you get zero or just one healer, while opposing team gets 6 or 8…
Sadly it seems Blizz doesn’t care about battlegrounds at all anymore. I doubt if anything will change with that regards in Shadowlands 
PS. if you want to check about the multiboxing issue - look at this thread:
25-40man multiboxer(s) in BGs - is sooo much fun !