My newish tub is on an Ez Pad and so far so good. We actually did a gravel box/Ezpad combo. We dug out a small level trench in a square 8'2", then put in treated outdoor wood, attached at the corners and staked it for stability. Then we put down landscaping cloth and about 15 50# bags of pea gravel to get a level surface. Then we put the Ezpad on top of that and adjusted for leveling. We chose this route so we didn't have to dig out too much for the gravel, but still have as level of a foundation as we could. So far no problem. We plan to check with a level intermittently.
Also, fyi, when I did my research on this is seemed that EzPads were more recommended than the Handipads.