Wow, it sucks that no one replied to this.
Yes, going fully poly/poly is not a good idea. The resistance to rotation needed for the car to roll, causes spikes in the wheel rate as the car rolls, and that makes traction unpredictable.
One exception might be a fancier multi-piece poly bushing, which uses lower durometer material on the outboard edges of the bushing to help accommodate roll. I don't have any hands on experience with these.
I am running an UCA and panhard bar from Spohn, with a poly/del-sphere combo. The Del-sphere joint is essentially identical to the roto joint. The concept is great, but Spohn's execution is terrible. They paid no attention to important things like fitment between parts. For example, the heinously loose fit between the UCA mounting bolt and the center of the sphere, or the two adjoining spacers. Or, the fact the the spanner nut to tighten the del-sphere will actually make contact with the sphere, instead of just pushing on the delrin cup, due to a lack of concentricity on the nut's features, and improper chamfered relief on the nut. How about that they didn't even bother to put a protrusion on the side of the UCA to capture the factory tabbed nut, keeping it a one-tool job like from the factory, and forcing you to put a second tool on the very shallow headed factory nut.
Spohn is a turd of a company, do half-assed work at best, and do not stand behind their products.
I'm interested in finding some hands on opinions about UMI in general, and their roto joint. I've exchanged a few emails with them, and they seem responsive and helpful, but I haven't put my hands on any of their parts.
Even now with the Del-sphere/poly combo on the UCA, I can still get a hint of wheel hop in certain situations on wet pavement. Annoying. I think maybe past wheel hop has damaged the rubber LCA bushings. So, I've got an eye on the UMI LCA's on-car adj. Roto/Poly combo.
Hopefully someone else can chime in with an experience!