The difficulty lies in water chemistry, imo. They are frickin impossible to spawn, much less get a decent hatch out of unless you're flooding the system with RO. That's what I had to do to get fry.
I believe the long standing belief, which, to some extent, makes sense to me, is that for softwater fish, especially blackwater ones, eggs tend to be less resistant to "normal" microbial attack without the care of the male because the black soup and the subsequently impractically low pH only allows for the culturing of very acidophilic microbiota, which tend to be somewhat slower, metabolically speaking, and just less competitive in neutral aquarium conditions.
When you couple the fact that these eggs are designed to deal with slow growing microbes plus, a lack of microbial activity altogether, you see where the problem might lie.
Of course, if the eggs are actually infertile, not just fungusing, that's a whole 'nother issue.