The cassette toilets can and do stink if you do not clean them out often enough or add the toilet chemicals to the cassette toilets.
I have a portable cassette toilet that I keep in my van when I'm traveling in my van and need to use the bathroom when I pull over somewhere without a bathroom.
It does stink a little bit but I empty it as often as I can and should and add toilet chemicals to it to keep it clean and it doesn't smell too bad.
But if you do not keep the cassette toilet clean or use the chemicals like you should or empty the cassette toilets often enough then it will stink.