I have a Kubota
B6000 (12hp) with 2 pt 3ft wide factory fitted tiller. [Now my second tractor and only used for tilling duties].
As has been mentioned these have curved "knives" which cut into the ground. Tractor has 3 speed pto for tiller work.
On heavy clay un-tilled sod, I have to make several passes with tiller at lowest speed. The nature of the cutting action does not always lift and turn the sod out. It could be 4 or 5 runs, increasing pto speed every other time before I would have a workable soil. It is only the top pto speed which pulverises' to a reasonably fine tilth.
The advantage is that the design requires less hp to operate the tiller and by their shape are not affected by stones and the like as with standard tiller cutters. The tiller will bounce over stones and bury them slightly rather than fighting and bringing them to the surface.
Having used a 7ft "Howard" rotovator behind a Ford 4000 for many years, they are nowhere near as good for producing a fine powdery tilth. However I believe the design and operation makes them more robust and less prone to operational damage. That's why they can be directly bolted to pto housing and do not have shear pins or slip clutches.
Don't be afraid to buy and use one, but beware that they do not operate in the same way and do not produce the same results.
As always, it will depend on the reason you want to till and the finish you require.