Has he really ever been targeted enough to say that he has hands of stone? He has occasionally been on kickoff returns and I seriously doubt that job ever presents itself to anyone who cannot catch the ball.
You can say he only can run a go-route and that he had never had the opportunity to develop chemistry with Tom Brady, but I don't see the body of work to suggest he has hands of stone. Lots of guys would have that label if they played WR two snaps a year, running 50+ yard routes each time, and being thrown to in a career what, twice? three times?
Unless you have seen something in open scrimmages I'm unaware of, the Slater badmouthing is excessive IMO. He more than likely is not, nor never will be, a quality NFL WR, but that's just an assumption based on the fact that he hasn't been used that way. And why would he be, if he's a pro-bowl caliber special teams standout?