I had an old Z250 many years ago with a well worn throttle cable - didn't need a throttle rocker! And when the cable snapped I had to do an Ogri - pulled the broken end of the cable to accelerate for 15 miles to get home . That hurt!
After my camping trip to Dorset, all i can say it was great taking the pressure off my wrists on the motorway. But on "A" and "B" roads it was a nightmare. I had to take the thing off. Hitting a roundabout without closing the throttle down was bloody dangerous to say the lease. If the hand grips were a couple of inches longer i could leave it on at all times.
Yes, you are prob only best to use it (them, in my case) when Ona motorway or a long stretch without interruptions.I have nearly caught myself out a couple of times approaching junctions.
It is called a 'crampbuster'. I have been using one for many years with no issue. Cheap and it does help minimize wrist effort to hold throttle open. And not a safety issue.