What in the holy hell is the downside to Sparky?
Honestly, I don't think it will be a big deal and won't be any different than letting a PEKKA/Golem/Prince run around the map without a defense. In the video it looks like it has fairly low HP that can be easily taken out by the Skeleton Guards or any other cheap card that has mobs with it. You definitely will have to deal with it, but that is no different than if any of the above cards are played. I think it is just the initial shock of watching the video and what it can do, but they were playing mostly the new cards and not the others like Skeleton Army, Goblins, etc. which I imagine may counter it decently. If you let your opponent build up a push, then yeah, that will be a hassle to deal with. But that is no different than any card out there currently.
I definitely see the salt mines kicking into overdrive with this card though.