i have both what are probably the best fixed hubs in the world (phil wood) and formula hubs. i definitely have more miles on formula hubs than the phils. i've never had a lock ring problem. ensure you are using a good quality cog, with flat faces, make sure the cog is threaded on fully, and there is enough exposed locking ring threads. next, use a good quality lock ring. thread it on by hand carefully, use plenty of grease, and ensure it sits squarely against the cog, no gaps. tighten down securely. recheck tightness after a few hundred miles. if you do this, you'll be trouble free. formula hubs are inexpensive, use easy to find and replace bearings and are generally decent quality.