That one is pretty easy. Let's look add the current meta:
- Top: Juggernaut (basicly tank after 2th item)
- Jungler: (basicly tank after 3th item)
- Mid: Lategame dps
- Adc: Hyper carry
- Support: Tanky enage support
Lb is a champion that relies pretty heavy on snowballing but this comes with a hugh risk. Early game Lb will get outdamaged pretty easy by the toplaner & jungler. She might be stronger midgame but than the enemy midlaner & adc will take over. So while playing as a team their isn't a lot an Lb can do if it's not an 1v1. Her job basicly start to depend on how hard she can snowball the midlane. Maybe getting one lucky with one nice dive on the botlane. In the lategame she basicly is depended on getting to the adc or midlaner or die trying. This is how a assassin suppose to work and she probaly one of the strongest assasins that is able todo that. Mostly because she has decent burst & aoe damage. If the support was squishy too their might be a reason to pick assasins again but so long she can't oneshot 3/5 members on the team you probaly won't see her often. Playing an assasin was always a gamble when it comes to damage. This combined with the fact that the 5.11 changes spiked her skillcap it's pretty dumb to master her over something like an Azir that has more dependable dps to work with.
Yasuo is different because he can still manage to shred throught the tankline because his ult that aplies a lot of armor penetration. If the op picks are ballanced out I probaly go to ban him again. Not because I can't play against him. Just because I known my teammates will feed him. And even when he strungles a fed hard cc guy can always get him back in the game with just one ult. Yasuo is in that area less of a gamble when a teamcomp has been build arround it.