I have a mmm, not a rmm, but I think of one of the advantages of a rmm as being the ability to back under trees and bushes, into corners, and onto the edges of ponds and creeks. I am certain I would do that if I had a rmm.
I will report here some very expensive lessons I have had backing up with my 2910's 72" mmm. To explain, I'll define some terminology, since I dont know how to draw pictures. Each of the 3 blades on the mower rotates in its own circular chamber. At the rear of the mower, this circular chamber is formed by a semicircular wall or "baffle". (At the front of the deck, the rest of the circular chamber is formed by the deck housing itself.) Thus the blades rotate within these circular baffles at the rear of the deck. Switching to the outside of the mower, the lower edge or lip is reinforced by circular tubing. But this edge reinforcement is only on the front and sides of the mower; it is omitted on the back of the mower deck housing. Thus, on the rear of the mower, the unreinforced blade baffle is the only thing between the blade and the outside environment.
What has happened to me three times is this: There is a little root or stump or rock protruding an inch or two out of the ground. Going forward, the reinforced lip on the front of the deck bumps over the root, the blades may or may not shave it, and then it bumps slightly over the concavity of the rear baffle wall. No real problem.
But then I back up a couple of feet. This is usually because I am jockeying while trimming around around a tree. Backing up, the little obstruction hits the convexity of the unreinforced baffle and dents it inward. Then the blade starts hitting the dent and ripping the metal baffle wall to shreds. By the time you figure out what has happened, due to the loud clattering, you have damaged the baffle and your blade.
Three times this has happened, and twice I have had to have the dealer repair it with hammers and welds. (He did it for free the first time because it happened the first day I owned the tractor.)
This infuriates me because I think it is a design deficiency, but Kubota doesnt agree. I think a deck that costs almost $3000 and weighs almost 500 lbs. shouldn't break on a tiny root when you are trimming around a tree--a standard mowing maneuver. The whole problem could be avoided in any number of ways. One is to have extended the reinforced bottom edge around the the back of the deck. Another is to have filled in the cavity on the outside of the baffle wall so that it could not dent in a rear (or forward) maneuver. I ended up paying the dealer to weld reinforcing plates on the backsides of each of the three baffle chambers, as a remedy to a problem the Kubota should solve.
The moral of this long story is: inspect the design structure and reinforcing on the back of your rmm or mmm before you start backing into things. Ask yourself whether your baffle would hit and dent if you hit a 1" pipe or stump sticking out of the ground. Then be careful.