The Lava Shells are light, considering part of a GOOD hot stone treatment is having the weight of the stones do the work, rather than the therapist having to do as much work, you immediately lose that benefit. Also the heating stuff in the shells is a consumable so you have that continuing cost every time you do a treatment.
The heating stuff in the shells provides continuous heat, so no running back to the stone heater to grab a hot stone.
I think the lava shells can do a nice spa style relaxing warm massage, but for therapeutic results I'd be looking for something larger and for some cold to be included, be that hot and cold stones or heat pads and ice packs, the temperature is what leads to significant therapeutic results whatever medium you are using to deliver the temp