I've had a similar failure rate: I've got two bikes with SunRace freewheels that are doing just fine, but I've also had two of their freewheels give out on me - they stopped spinning freely, then developed extra play, then started to do what felt/sounded like internal grinding, at which point they were replaced. The common factor is that the freewheels that suffered were both used on commuting bikes at sub-freezing temperatures, while the two that are still working are on bikes that I don't take out in inclement conditions, so it may be that there's something about the metallurgy or factory lubrication that's not designed to tolerate those conditions.