I've downloaded both of those demos and haven't had any problems with them. I played the Unit 13 demo a lot before the full game was released. Haven't played the Hustle Kings demo as much though. Only other demos I downloaded were Rayman Origins and Little Deviants. Those two haven't given me any problems either.
Hustle kings work fine. People complain about freeze ups on unit 13. Honestly I havent tried it But Motorstorm Rc Frozed my vita once. I am thinking its a bug in sonys firmware and not the demos/games there self. If you run across a freeze hold power button 30sec then restart vita.