Yes, I remember reading this back in the day (when Maul just popped up in front of Savage in Clone Wars) and thought "surely the writers are taking the piss". I mean, you just need a lot of things below the waist; even if it's cauterised and sealed, and you technically have a functional cardiovascular system and brain (and of course didn't die from the massive trauma because Force Hate) I really don't know how you're sup. posed to urinate and defaecate, as a basic starting point, and therefore eliminate toxins or deal with having eaten or drunk anything. I think "Force survival trance and Control-based biofeedback" is doing a hell of a lot of heavy lifting here.
(The Doylist reason is, of course, that Lucas said "let Maul return, somehow" out of nowhere and Filoni said, "how TF, you cut him in half?!!" but then had to make it work, and was presumably just doing it through gritted teeth. But then did an amazing job with it! Post-bisection Maul is a 100% better character despite having only 50% the body mass.)
One obvious point - Maul therefore spends the last two decades of his life not being able to have sex (at least conventionally) and this is probably pissing him off. And thus making him more powerful.