That's not the experience I've had. I play with a couple of different clubs regularly. There are one or 2 people that use pre-made builds. Otherwise, it's all custom builds. We go up against teams stacked with pre-made builds and win the majority of those games. If a team is regularly losing to the pre-made builds, that's going to be a difference in skill between the teams.
I'm definitely curious how the new McDavid build is going to affect this though. Up until now, the biggest issue I've had with those builds are the skins. The McDavid one is pretty scary so I'm looking forward to seeing how games go against those players.