Doubles are less affected by tiers and stat listings, too. This lets people use more of their favorites without having to worry.
Though there are a few pokemon that seem to create a much bigger problem then they ought (talonflame is the only one that seems to cause a LOT of trouble, other things like garchomp are very manageable).
I've only fought the same style of team a few times, and they were all from japan (garchomp, rotom, tyranitor, togikess)
It's very varied, and I've only played a few games where I won and thought "there was literally nothing he could have done to prevent that", and the opposite, where I felt like I lost and couldn't do anything about it. Most games, upon reflection, you realize you could have made way better choices.