I've used a Piranha and had so-so results. I've heard the Harley is a better unit.
The Piranha that I used was slipping the clutch to a point of stopping the roller until I lifted it. The dealer claims to have used it since w/o issue or complaint. In fairness, the dirt that I was in was very hard and heavy clay, and I was tilting and often angling the unit to pitch. The knobs would build grass and dirt on the tips. I'd clean them and in a metter of feet they'd build right back up. When it worked it did a nice job. I'll also add that I quit when teh roller started smoking at the one end bearing. This would be the LH side of the machine. Dealer said all that he found there was maybe some twine wrapped...
The Piranha last I checked is also not ASAE Cat 1 compliant which is important to me. It is a beast and having an Imatch to hook up the pins sure speeds changing of implements. I still have a lot of work with my folk's driveway (1000' plus) and when they build their house a lot of finish work there also.
If you have any points on why the Piranha would be better, please share.