It's important to remember how imbalanced the Preseason is always bound to be
It's been the same pretty much every preseason since I've started playing (back towards the end of Season 2). New items introduced, some items removed, other items buffed or nerfed, new jungle changes, new mastery changes, etc etc. These huge, sweeping changes all coming in at once are obviously going to throw the game balance out the window.
Yes there is a shit ton of imbalance in the game right now with these current preseason changes. Yasuo can get to 100% crit chance with 1.5 items. AP burst mages are sorely lacking options in terms of masteries, and their items have been nerfed while MR items have been buffed. The many kit changes to the Marksmen as well as the Ferocity keystone masteries have made them near broken, or incredibly hard to deal with.
It's our job as players to give feedback on these kinds of imbalances, while Riot observes for themselves just how imbalanced it is. I'm not saying NOT to complain about the imbalance, but I do think it should have been expected. The preseason is always chaotic.