Well, there was an international event last year if you don't remember, Imagine Dragons did a song for it and everything, and this event pretty much proved Korea is MILES AHEAD as far as meta is concerned. If the Korean competitive meta believes Rumble is strong enough that he deserves being picked/banned 50% of all games, he probably deserves his spot as a strong pick.
In fact, most of the overwhelmingly strong champions in the competitive meta are not what the NA community complains about, Kassadin, Lulu, Rumble, Cassiopeia, these are the champions to be afraid of, but I keep seeing Yasuo threads in this day and age.
Solo-queue winrate is an indicator of well does a champ fare when played in low elo, since they make up the majority of the population, and I agree, it does not accurately indicate the real power of a champion, however, competitive statistics are an excellent indicator of a champions potential since it tells you how good the champion is when played by the very best players.
If you somehow don't think neither of those factors really prove that a certain champion pick is a strong pick then what do you believe in?, personal bias and experience?.
I have NEVER seen a good Rumble, few people actually play him, and even less people can master him, but I'm pretty sure he's a strong pick, because he's one of the most relevant competitive picks and his diamond winrate is huge.
Finally, Hearthstone is a young game, metas take a while to develop, specially in young eternal formats where each expansion pack changes like 30% of the card pool. Take a look at MTG Legacy meta, it has been there for such a long time, the relative card power is well known and you can accurately judge most strategies according to the meta, there are archetypes, counters to those archetypes, sideboard options and so on. I'd say a 120-champion game with like 8 champions released per year is old enough to develop a solid metagame.