Shyvana is a typical melee-duelist. She doesn't bring much to the table besides being able to farm the jungle REALLY well - however a LOT of other junglers do this as well. Hell even Braum with a Cinderhulk can farm the jungle decent. If she tries to gank you, you can just walk away. Unless you stand there trading autos with her, the W will run out rather quickly, and she won't be able to get any resets on Q, much less make use of the bonus damage on E. She can be a fairly-beefy frontline with Cinderhulk and can still provide rather decent damage with a BotRK/Titanic Hydra, however aside from diving in with Dragon Form she has no real way of closing the gap and can only sit there hoping you turn and face her. She's not a super-tank, and doesn't provide safe carry-esque damage, so she's a very niche pick. As people have said above, it wasn't her that decided the game (provided the enemy team had a jungler that actually did something). Sure if you let her right-click you to death she will win, however she thrives on counter-jungling and being able to powerfarm really well. If she goes lane to lane just to tax, the enemy team will be FAR weaker and it's pretty much a 1v9 for her at that point.
I've played against many Shyvanas, and 90% of the time it's them powerfarming and flaming their team for being "garbage" whilst I ganked the lanes consistently and was able to get them ahead, which puts further pressure on her as she desperately needs farm to be relevant. Rek'Sai can rush level 3, gank, grab a Tiamat, and be able to keep up with Shyvana's farming, whilst having more sustain and still being relevant to the entire map.