Personally, I think it's luck of the draw. I have worked out, (roughly) that I have had one problem every five years with the same two magnetos over a period of 35 years. This includes the points plate coming off its taper and the earth kill brush breaking and causing a permanent earth, so no spark. The first took half an hour to fix, and the other about two minutes. The only other problem has been difficulty starting when the mag got warm, which is almost always the condensor failing. Not once have I had to recover the bike due to it's failure! On the other hand, I put a brand new Boyer on my 750 Bonnie and had a nightmare, the bike started easily, ran perfectly for about five miles and then complete ignition failure. After about twenty minutes it would start and run for another five miles and conk out again. Working on my long held belief that electronic ignition either works or doesn't, I went on to spent lots of time and money trying to get to the bottom of it. Eventually the unit went back to Boyer and they sent me another saying the first one was faulty. (The service incidentally was fantastic, I posted the old unit on Tuesday and had the new one back by Friday. No charge either). The other half had a new VW Golf which would cut out completely, and then start after anything from a few moments, to a few hours.
I look at a mag as a single item, one which is full of sparks and will usually give you plenty of warning of a developing problem. Changing it is an easy and relatively quick thing to do. As such, if a problem develops that I cannot rectify by changing pick ups and brushes, leads, caps and plugs, I put my other mag on and send the origional to a reputable restorer. Being a single and compact ignition unit, it is not hard to find space for your spare if a long journey is ahead of you, which is another plus point. I think this is the principal with light aircraft which have two complete systems that you can switch from one to another. No doubt an electronic jobbie will have your engine running better with its progressive mapping, rather than the advanced or retarded and nothing inbetween with the auto advance.
My advise would be fit what you want and then rub a rabbits foot!
.......Good Luck!