I hadn't thought about it like that before. The part that had me thinking about it was that they are more compact. I can't see pushing a 6' rotary cutter on a F series where I could see it pushing a 6' flail
You're right. You wouldn't want 6-8ft of rotary deck out front of a commercial mower frame. That's why they make flail decks for them. They're effective, lighter, more compact, more maneuverable, and have a high degree of success in the European market.
To my knowledge, the following suppliers are all that's available to us here in the states, and aside from the Perruzzo offering for the Grasshopper front mount ZTR, they're light duty units are all geared toward the front mount tractors you speak of (Kubota and John Deere F series, etc.). They are:
-Vrisimo (I think that's how you spell it)
-Perruzzo
-Trimax
There is another manufacturer who makes one for Wright brand ZTRs, but again... 'til they present a better product, I won't even consider them.
Trimax is biased more toward equal quality finish mowing as well as rough. They're premium quality, have exclusive options and adjustments, and also command a PREMIUM price tag.
Perruzzo has nearly the same finish capability with the right flail selection, but doesn't have all the fancy hardware that most flail users wouldn't require... at about 10-25% less cost (depending on options). They also seem to process the really tough stuff (5-7ft tall vegetation) much better than the Trimax.
Vrisimo is the more rugged purpose built rough mower of the three, at about the same cost as the Perruzzo.
Basically, if you're focused on finish cutting with some added rough cut capability, buy the Trimax. If you want a machine with the broadest application range, go with the Perruzzo. If you want a tank of a unit and are only brush mowing... try the Vrisimo.