I think the most common problem with FW fish, is people have more of a tendancy to overstock and overcrowd thier tanks. I have had FW tanks for about 18 years now and I've only had one case of finrot in the tank and that was because I came up with a brilliant idea
(I was very young at the time!) of mixing Tropical fish with local coldwater fish.
In my Marine tank I've had a nasty case of Marine Velvet that nearly whipped out all my fish because I didn't Q them.
Fish on the whole are generally very hardy and disease resistant, but we keep them in an environment that is less then ideal and very stressful.
Having said that, FW fish are also begining to suffer from a lot of inbreeding to get that perfect fish with the bulbing eyes and extra long flowing useless tails, and a lot of hormone usage that is redusing the health and life expectancy of FW fish.