Land-based toilets are very easy (and clean) to fix. The bowl, of course, is disgusting, but there is nothing in the bowl that breaks. It has a built-in siphon, and it works by water being dumped into it. In fact, if you pour a bucket of water into your toilet bowl, it will flush. It is a simple design and there is nothing down there that needs any maintenance. There are things in the tank that break, but the tank (which is just an automated bucket of water) only sees clean water, so repairing it is a clean (and usually easy) job.
Boat toilets might look like regular toilets, but they operate completely differently. There is no siphon and no tank of clean water. Nothing is simple, and there are dozens of things that can (and do) break. And all of those things spend their lives sitting in poo. Some are located in the toilet bowl itself. The other half are located between the toilet and the holding tank, where the poo goes to rest before it is emptied into the sewer system or (if you are far enough for shore) the ocean. There are valves, hoses, pumps, levers, tanks...all filled with poo, and all in need of maintenance from time to time.
I recently ventured on a long, disgusting journey of discovery in order to learn how to replace all of the parts that break. On the way, I saw some truly horrifying things, but at the end of the day, I became fully trained in the operation of our potty, which now works like new (and will hopefully stay that way for at least a few years).
I'll spare you the most disgusting images, but do you see that white valve at the bottom of the bowl (where in your toilet there is only a hole)? Well, I replaced that thing. You can imagine what the part of it you can't see looks like.