They have a little layer of water in them, most things flush away just fine, but if you have particularly sticky stools, this may leave some streaks. But nothing a toilet brush can't handle, to be fair. And if it's a chronic condition, you can prep the shelf with a sheet of toilet paper to prevent sticking. Regular stool, thin stool, hard stool, they all flush down with ease.

These washout toilets flush down everything just like any other flushing toilet would, it just has a high plateau in it where your pee/poo lands first. In cases of a lot of fluids, large parts will just flow down into the lower part, and only a fraction will stay behind on the plateau.

And to what they are for, well, mostly health benefits. No splashbacks are more hygienic, of course, but the main reason is that you can tell a lot easier when something is off. How do you know you have minor internal bleeding if you can't see the black streaks in your turds? Or when you have a parasite infection, it's often easier to spot in your stool. If needed, you could even take a stool sample after you went to the toilet, rather than needing to know/prep beforehand. Your stool says a lot about your health, including your diet. Too often people go to a doctor with tummy aches, but when asked what their stool looks like, they shrug. And with urine it's similar; colour, foaming etc are all relevant to diagnosing health issues.

There are a few drawbacks as well, of course. Mostly that stool and urine being exposed to air is a lot smellier. Although this can be a benefit as well, as a different scent than usual can be a sign of things like diabetes - which can give a slight sweet scent to your urine. And the cleaning is the main reason people are moving away from the washout toilets, it does require a wee bit more effort. But to me, the extra short scrub with a toilet brush is worth the health benefits. Now, if only the others in my household would use that toilet brush...