They are quite absorbent and have the advantage over the other mediums in being edible and actually good food for tortoises. Many recipes for tortoise diets include rabbit pellets. The disadvantage is that if you do not promptly remove soiled pellets they may be ingested or become moldy. They also seem to have a dehumidifying effect in a warm cage which can be a plus or a minus depending on species. I have been experimenting with this substrate in my large tortoise and Bearded Dragon pens and have not really found any drawbacks yet.
Most tortoises probably benefit from full-spectrum lighting.
It is possible that tortoises housed outside during the summer may get sufficient UV exposure to last through the winter, especially if they are adults. This whole area of UV needs and appropriate sources is much debated. In the past, fluorescent lighting has been the only source of indoor full spectrum lighting, but I understand that there are now incandescent full spectrum bulbs available. In addition to full spectrum light, the tortoises need a heat source. I like to use a combination of top and bottom heat sources.