either type spring can break and either type can last forever. i've read that properly made flat springs are more reliable than properly made coil springs but i can't recall who said that. i doubt one can make that blanket statement with any credibility. i've had guns with both type springs (not just talking double mainsprings but all sorts) break. with today's manufacturing technology even in parts of the world not known for metal work it's no great trick to turn out properly heat treated parts from good materials. anyone who can do simple math can use a modern computer controlled heat treat oven and get xlnt results.

one guy i work with has two cz's: a 28 and a 16 and just loves them both.