Speaking about who's more powerful is meaningless imo in some kind of situations, this is one of them. Even if someone is weaker he can always defeat the stronger one by catching him off guard or if he has an advantage or something like that, that happens in lots of situations, either in anime or in real-life. That's what happened with Byakuya. Byakuya went through some character development in SS arc, and it continued during the next 2 arcs. not to mention the filler arcs. He developed a soft-point for Rukia, which eventually became his weak-point. The Quincy likely did some researches and studied the Shinigami before attacking, I mean no one would enter a war or do a raid without studying the situation and his chances of winning, or do some counter-measures. They obviously did and that appeared with driving Ichigo to HM and the Bankai stealing, who knows what's coming next.
I think Kubo is trying to make the Quincy look like some unbeatable group who are too strong they can annihilate Soul Society, but in fact if they can be defeated by Bankai then they're at the level of the Espada, there might also be strong guys and weak ones too, and their leader is at Aizen's and Yama-ji's level.
