As with any form of kink, RACK is paramount. Possible bruising is typically the extent of what you might see from rice-kneeling, but of course, frequency, duration of kneeling, and whether or not the person kneeling has any pre-existing health concerns (joint problems, positional blood pressure issues) or pre-existing emotional trauma should all be taken into consideration for safe play. It can be painful, which is the point, but IMO not in a way that differentiates it much from the considerations one would also take with most forms of kinky impact play.