The banner autobalance thing is a vast improvement over the previous autostack system, but it has this one problem, at least on the NA server, and from this issue comes all the complaints I think: when 20 or so ATS players join a single server and get placed on the same team, it drastically unbalances it. Putting any large amount of half-decent, half-coordinated players against the cRPG mob will almost always result in victory. This gives that team large XP/gold bonuses. Thus, other team either rages, complains, or goes to a different server. ATS is the only clan I've seen large enough to consistently do this, since most other NA clans (including my own) usually have less than half a dozen players on at any given time on a single server. LLJK don't count because they are goons and worse. I don't know how things are in Europe, but I'd guess that there are enough big clans there to keep things competitive.
This isn't ATS' fault, its just the community is small. Autobalance works by numbers, it can't account for player skill or teamwork.
How to fix this I don't know. Probably the easiest, to be frank, would be, when lots of members of a single clan are on, to split up into 2 "squads" with different banners, and so hope you get on opposite teams and can have more organized battles for everyone. But that depends on the clans, not any coding the cRPG dev team can do.