I don't do any breeding (yet) but like to keep abreast of breeding trends, at least as much as an amateur paphnut can. So I am definitely talking out of the rear. As far as I know, most if not all of the primary grexes are viable. I think there are species groups that are incompatible (eludes me right now), and I'm can't recall certain primaries like parvi x insigne complex, but then again, who the @^$$^# would make those crosses anyway?
Which brings up an interesting point. Paphs have different chromosome numbers, but their primary grexes are usually fertile...whereas prior to the ploidy revolution, phrags hit a total dead end after 1 or 2 generations.
You tend to hear more gripes about paph infertility when it comes to white complex hybrids (a quagmire of genetic material).
While I'm rambling, I might as well put forth my cynicism towards the RHS database. There are maybe a half dozen or dozen phragmipaphium hybrids registered...I am not convinced that any of them are "real". Except maybe for that Hanes Magic 'Bion' entity of the 1980s, but the only thing I've seen was a poor-quality photo. Alright, I should probably quit talking now.