Amongst my friends/acquaintances I've seen two magnetos fail. I don't trust them.
Some of the clone shops turn out really high quality clones. SXK/Infinite and EHPro/wotofo come to mind. I have a stainless steel SXK praxis clone that is identical to the authentic aside from rhodium plated negative contact and top plates. Threads are smooth as silk, maybe a little better than that of my authentic tarsius 4Nine.
The biggest two things to look for in a mech (IMO) are:
-ease of switch disassembly
-ease of battery removal
You have to maintain the switch contacts, so it (maybe they) should be easy to remove.
I feel easy battery removal is self-explanatory.
Most mechs up on FT have discussions on whether or not they are a reasonable buy.
I would also say that a good SS panzer clone is a mech everyone should have. They are easy to maintain, rugged, and due to the extra mass, have a pretty low resistance (just like thicker, lower awg wire has less resistance/mm more massive mods have lower resistance, and thus less voltage drop). Skip the ceracoated blackhawk models unless you know it comes from a factory that leaves the threads unpainted. As much as I love copper, SS is a lot more practical for a tube mech; not only is the outside more resistant to damage, the threads are more likely to stay intact over the course of their lifetime.