In that case, I would say that if someone is coming up with their own grain formulation for fermented feed and not paying attention to the nutritional content of each component and how it all adds up, then you could end up with inadequate nutrition being provided to the birds. Of course, that holds true for non-fermented feed as well, in instances where people are not using pellets/crumbles/mash or are "supplementing" those feeds by, for instance mixing in scratch or corn or whatever other addition they seem to think is necessary and throwing off the "wholesomeness" of the food. Given that FF is generally composed of the same ingredients that a given individual would feed dry, I cannot conclude that FF would be nutritionally inferior to dry food at any point unless they are using a component that begins to lose nutritional value when wet, which I cannot think of off of the top of my head but I'm sure exists, or a component that is likely to be converted to a toxic substance once fermented. Again, I can't think of an example but I'm sure such things exist. If an inappropriate starter is used and alcohol fermentation ensues (rather than lacto fermentation), that could be harmful. Fermenting your feed in crockery that isn't lead free or in metal containers could also make the feed harmful, as well.
Generally, anything I see about downsides to fermentation have to do with possible contamination, either by chemicals or biological agents, or inappropriate type of fermentation, rather than anything having to do with the nutrition.
