It doesn't matter if its ARAM or QM winrate. It's even more inconsistent with ARAM in which heroes are randomly assigned to people and the matches are anything but balanced.

Burden of execution only goes so far. The top 6 highest winrate heroes over the past week have extremely high "burden of execution".

Finally, there's having a high WR with lower popularity or been niche for certain maps (old Garden Terror TLV sounds a bell?) and then been practically the same maps in the case of ARAM.

There's a massive difference in the sense that ARAM is completely randomized team comps and the fact that you choose between 1 of 3 heroes. You or your teammates may or may not have experience on that hero. Further dilutes the winrate claim

Also to refute your claim that BW has one of the highest winrates. She's behind Ana, Morales, Lili, Malfurian, Anduin, and Deckard. Not that it really matters because winrate is a terrible metric.