I can imagine many points where this kind of signals will lead to distorted results. (e.g. video goes viral because of other thing than your game). Do you have also experience when it went wrong? How did you handled it?
But, it's harder to imagine how Steam wishlist counts lead to totally distorted accounts. Unless someone is botting your Steam store page and the wishlisters don't exist, they're all still potential customers, and 5,000 wishlisters means something and 50,000 wishlisters means something.
There's lots of conversation about 'quality' of Steam wishlisters between developers. For example, Steam Fest is an event that Steam runs every now and then with game demos and there's very prominent 'wishlist this game' buttons. It's generally accepted that these wishlisters are 'worth less' than if they come from YouTube or PCGamer or even other sources upon Steam. So if 90% of your wishlist count comes through Steam Fests, yeah, you probably do have a distorted metric to work with. But no, I haven't experienced this myself.