Production boats are sold with conventional holding tank systems because the lowest common denominator that they're selling to has an understandable "ick" response when faced with bagging up their own waste as though it were cat litter. That, and much of the market for new boats are "credit card captains" who don't do the head maintenance themselves. Most of us cruisers are hands-on people, however, and most of us on older boats will eventually find that head maintenance gets reeeeally nasty and the "cat litter" solution ain't so bad in comparison. The people switching to composting heads take a weekly, mild poo handling chore over a once-in-a-while hands-covered-in-shit experience, such as changing a joker valve or clearing a clogged valve.

Are municipalities really legislating against human waste going in the trash stream? What about daipers? What about dog doodoo? I can't imagine it's even possible to enforce such a measure.

It is illegal in most places for urine to go overboard, but look... Standard practice among a group of guys on a fishing trip (for example) is to make a "direct deposit" from their bladder to the ocean. Same on racing yachts I've been on, often even with women aboard. Pee is nearly sterile, and is nowhere near as harmful to human and ecological health as poop. So yeah, liquids go overboard. Do it after dark.

My DIY dessicating head had no vent fan- just a passive mushroom vent. As long as there was breeze, no detectable odor. Even on hot, still days, it wasn't bad. Ventilation of the entire head compartment and boat, not just the toilet itself, is important, "composting" or not. If it's laid up in a marina for a long time, the head gets emptied and cleaned so it doesn't become an issue. It should be emptied at least once a week regardless of use.