We put a C-head composter on our Seaward 25 a couple of years ago, and it was one of the best improvements we did on that boat. Our Catalina 350 has a pretty nicely designed electric head setup on it, so I recently replaced the Wicox Ctittenden unit on it with a Raritan Sea Era replacement base unit and remote water pump. Replacement parts are few and far between for the Wilcox units, and ours needed some fixing. New boat to us, and I have high expectations for the electric head, but if it doesn't work out I wouldn't hesitate to replace it with another C-head.
So, I guess I'm saying it depends. If the boat has a well designed system that works and just needs some repairs, go with what you have and fix it. If no matter what you do, the system still has major drawbacks, then put in a composter and I doubt you'd ever regret the choice. We could go a month on our Seaward without ever needing to worry about pump outs (just deal with getting rid of the urine separately). That does give you a lot of freedom. Never had any issues with odors, bugs, or any of the other horror stories you read about.