Honestly, I don't think these two things detract from the idea that much. The cost and safety are, in my opinion, the biggest benefit. The amount of nuclear waste in the world is actually fairly small, and long term disposal isn't as big a problem as it's made out to be. The fact that these reactors could potentially be built cheaply and are immune to melting down is the real selling point.
The problem is with the psychology of this. If the scientists are incorrect on the fuel efficiency, what else are they incorrect on? Maybe it can melt down, maybe it can't be built cheaply. It's just fuel for fear, uncertainty and doubt.
Shoreham was closed because the oil industry spread propaganda to activate and empower a NIMBY crowd. It had very little to do with real risks.
