Yasuo as a champion

For once I'm not going to go on a rant about Yasuo, but I'm genuinely confused... When is Yasuo weak? He has an oppressive laning phase (normally if you walk up for cs he just runs you down). His mid-game is strong in the sense of IE + Shiv / PD, so he has 100% crit and shields. His late-game damage is nuts and I'm sure we've all experienced his late-game potential with 100% crit, conquer true damage and armor pen from his ult. I just honestly don't really know when he's "weak". One thing that I personally will not understand is why he has armor pen with his ult (so you can't build armor against his early) and he normally brings conquerer so even if you build HP vs him he'll go through that too. I'm not saying he's a free win champion, but I just don't understand why he is strong all throughout the game.