Not quite the bottom of the heap...
There's one that's worse...Just as there are differences in the quality of various makes/models of cars, electronics, home appliances and just about every other manufactured item, there are differences in the quality of the various makes/models of marine toilets. Cheaper, less durable materials vs higher priced, better quality materials...higher tolerances/lower manufacturing standards and methods (less expensive) vs. closer tolerences/higher standards & methods (more expensive...lower quality control vs higher when it comes to what's considered "acceptable" to go out the door. All these things determine the durability and reliability of any product, and marine toilets are no different. Cheaper materials can cause a pump cylinder to crack more easily or be more susceptible to being scored by sand and salt than one made of more expensive materials. Lower quality control standards can mean differences in how well the seals seal--the pump wall may be thinner or thicker...the seals a micron different in diameter or shape...higher or lower quality rubber. Molds and dyes also wear out and require replacing...another mfrg expense. The more worn they're allowed to become, the more difference there is between the specs and the actual products. Those are just some of the things that make the difference between a good quality piece of equipment and a "disposable" piece of equipment. Other factors include employee relations--poor leads to high turnover and lower quality work, good means people who are knowledgable and take pride in their work and who they work for.So the answer to your question, 'What fails? Is it soemthing renewable that wears out, or is it some other component that breaks?" is all of the above, or if you're lucky, none of the above. If you were to poll enough people, you'd get as many answer to "what breaks or wears out" as there are parts in the toilet, and as many people who've had -0- problems as people who've had lots of problems. The important issue is reliability...can you count on every unit to be exactly the same as every other unit? And the answer, when it comes to cheap toilets is, no. You could get lucky and get a cheap toilet that'll last forever...or you could spend half your time dealing with problems...or anything in between...it's a crapshoot. Durability and quality control are part of what you pay for in more expensive toilets. And rhe extra $75 or so for a toilet that has a reputation for durability and reliability is a small price to pay IMO.