I have a RISII FSP sitting in the drawer for awhile but I haven't put it together yet. I personally like URX better - after building a few URX uppers. The design is definitely stronger and more elegant. However, their GL compatible barrel nut is a PITA if you want to build a "cool looking" upper with the gas block under the hood.
The DD design is easier to install and repair, but the 6 bolts at the receiver end depends on integrity of the aluminum cut-out of the extrusion tube. Also the bottom rails depends on the 4 machined screws only. Both Troy and URX has the bottom rail held on by rails and spring loaded detent. I also find it interesting DD GL attachment point will transmit all the stress from the steel screw to the aluminum it is attached to. On the other hand, if a DD rail is damaged, it is super easy to replace it without unscrewing the barrel. This is very attractive from maintenance point of view.
IMHO, DD is a second best solution to KAC URX. However, URX is such a PITA to get, and even more of a PITA to install because KAC is not making the tools easily obtainable, especially with the GL compatible barrel nut.