I agree it's probably possible to maintain them well, but it seems it's still a cost. If nothing else, you have to pay for better maintenance workers, or more oversight of them.
But after reading about how they work, I have reservations on the design. Depending on maintenance for keeping sewer gases from escaping seems failure prone. Humans err (and get lazy). I'd much rather depend on plumbing codes that have been tested over decades and work "automatically" than depending on changing a cartridge.
What happens if a bathroom with a waterless toilet is abandoned or isn't maintained regularly?
It will float on top and prevent the water from evaporating.