I have had hully rollers for several years and have found them to be quite frustrating. Originally I had two sets, front and back, and that was a disaster. I never felt like I could get the kayak to be well-seated. So I got a "set-to-go" cradle for the front (my local dealer was no longer carrying Yakima), which worked OK.
I still find, however, that the optimum distance for using the rollers to load the boat is different than the distance necessary to hold the boat stable. I have a 17' Looksha IV, quite narrow bow and stern. I finally ended up with the rollers close together, so when loaded the boat sits on top of the locked rollers rather than being cradled by them. And they are close enough together that I can place the bow of the boat on the rollers, not the bar, when loading (I have a mini-van, not a car, so that makes things more difficult).
I have recently bought a Riot Stealth, which is shorter and lighter, which means that I can actually muscle it onto the roof of the van by myself (I'm a middle-aged woman with skinny little arms). The problem is that I can't seem to get the rollers to work with the Stealth at all. The stealth is only 12.5', and as I push it across the rollers and the width of the boat above the rollers changes, it slides from side to side across the chines. The boat does not have a flat bottom, either, so I can't really rest it on the bottom easily. I was ready to give up on the silly things, but reading other posts here, I figure there's got to be some way to make them work. I am going to try again with the rollers set wide, with a towel duct taped to the bar, figuring I can slide the boat across the bar and up onto the rollers as it gets to the wider portion of the boat.
Any suggestions humbly accepted!