Emberson had been trending up and looked like he was ready for a shot in the NHL the season before he was claimed. San Jose gambled he was ready to take a regular NHL shift, and he was.
Robertson has been trending down. His game has stagnated. He still makes the same incomprehensible decisions with and without the puck and just generally looks completely lost many nights. The offense never materialized (it was never really his game) but unfortunately he has been pretty weak defensively, too. If he were a UFA, sure, someone would likely take a chance and offer him a contract, but on waivers? That would mean there's a team that thinks there's a realistic shot that the guy is ready for the NHL. I sincerely doubt that's the case.
The reality is every team has one or more players like Robertson, a once-promising guy who has failed to put it together who gets the QO as a "last chance" deal.