the biggest con for a composting toilet, is in dealinq with the liquid waste from the body. Most require a separate vessel, and that can present a challenge. Almost all users are best served seated, that way one can usually point to the vessel.
But you still have to empty the liquid vessel. Some gents tote the vessel to the facilities, shoreside each am, or some store it in 2/5 gallon buckets to empty later or farther out. Some just dump overboard at their earliest chance, without regard to the regs. I say gents, as in all my years boating, I have only witnessed the male transporting the vessel. Ladies, feel free to chime in.
If one has IBD or digestive issues, the composting process can be overrun rather quickly.
However, a standard marine head is no prize for reliability, fragrance, ease of use, etc.
Name your poison ...btw, when installed properly the composting head really does not smell any worse than the standard vent when the wind blows back towards your nose. Some composting heads are MUCH better designed and manufactured than others, cost is not the pivot in this case. Some have an electric element and a turning mechanism to aid the composting process, if you have battery capacity this may be nice, however in my experience, ANYTHING that is supposed to move on a boat, eventually doesn't and viceversa. Things that move in the chamber can be cumbersome to clean/retrieve/repair.
Others will chime in, I am sure..as this is often one of the most hotly contested subjects.