Mags (especially the single shaft dual mag types) are probably the weakest link (statistically) on an otherwise highly reliable Lyco/Conti engine, but they are fine if you follow the 500hr overhaul periods.
As Johnm says, they do not require external power and that is a really super safety feature.
Electronic ignition would be more accurate, of course, but aviation history is not exactly overflowing with reliable electrics and electronics - I think because very few people with a brain want to work in a company which brings out a new product every 20 years or so
