Though I have no experience with these in the marine environment, I have one that has been in use at the lake house for the past 12 yrs and have never had a problem. We have no holding tanks(they are an option) and never had the need even with 7+ people normal use for the summer. AS for the fan I hardly usedit never had an issue with odors outside or inside, maybe a small wiff on a real muggy stifling still day.
I will empty the compost once a year (it could go longer) the compost had no odor and I spread on the flower garden. The worst was the bunches of Monarch butterflies that gathered around the mound and stayed for days, if that is a problem.
No plumbing problems, throw a sanitary napkin and just reach in and pull it out. better than cloged plumbing out at sea.
The only negative side to these was that unless your6.5+ft tall the comode is a bit high and we use a small step to rest your feet while siting on the throne.
It will take up no more space than the regular marine head with its associated hoses, valves etc. plus lots of room left over for storage(no holding tank)
All this said.... I have a jabsco, But it will be replaced with a compost when the time comes....