Yorick - super toxic laning phase got him nerfed. His late game is also abysmal. He isn't unplayable, in my opinion, but he is up for a rework soon, so I wouldn't put work into learning him right now.
Rumble - has also been in a bad spot recently, but has been strong and seen in the LCS before. Again, maybe a little difficult to learn, especially with cs'ing, but a decent champ. Usually thought of as an AP bruiser. Learning him might pay off; I don't think he's gonna get reworked soon, but he's probably the hardest to play on this list.
Vi - after nerfs, she's much weaker. Again, used to be very very strong and picked frequently in LCS. Her ult, though, is considered to be somewhat unfair because it lacks counterplay. She tends to be strong when her ult does an unfair amount of damage for how much cc it brings, and she's weak when she can't blow people up with it. It's a single target point-n-click guaranteed initiation, which has proven hard to balance. I think she'll be reworked eventually due to that, but if you like her, she might be worth investing in. I would recommend trying to run her in pick comps to blow up priority targets quickly and control objectives.
Sej - during cinderhulk meta was nearly 100% pick/ban in LCS play. Her strength is a very strong AoE initiation coupled with some AoE damage and tankiness. You use her with teamfight comps and try to burst the enemy team down quickly with a good initiation. Since cinderhulk got less popular, she's seen a lot less play. A lot of the community has been pretty angry that she got nerfs right before the cinderhulk item got nerfs, and then her individual nerfs stayed. It was a huge drop in power pretty quickly. She'll get buffed again if she continues to be weak. If you like teamfighting comps, Sej is a great champ, but again, maybe a little on the weak side right now. I would probably choose amumu over her right now, as they are somewhat similar (amumu is less tanky with more AP damage, usually).