It's not really much of a difference, and some live longer. It just seems more common I think. If you researched them, you know their original purpose, so then just the extras turning into pets is just something that happens. I don't think the original people noticed the issues, since the regular fish have the same ones.
It's like trying to prove a certain tail type is more likely to die early. Some are going to live a long time, and some other tail types are going to die early. So it's hard to prove, but it seems this way. Seems most people have good luck with these guys, but they go into it knowing and expecting problems, so are able to prevent them!