Drum brakes are less effective than disc brakes, that is true.
However, the reason most manufacturers still use them on the rear is that under heavy braking there is weight transfer to the front and the rears are unloaded. Whilst the rear brakes are only contributing around 20% of the braking effort, it is fine to have less efficient drum brakes. The typical drum brake design incorporates the emergency or parking brake and is a cheap and reliable braking system. When you change to discs you theoretically have more braking ability but in practice it is marginal.
If you upsize the tyres to 35" or above, you will struggle to stop with CJ brakes. That is a different ball game.
Servicing drums is quite straightforward.
Getting mud inside them is only an issue for mud pluggers, most Jeepers will never have had a drum brake slip due to mud. However if you are building it for mud, it is a valid consideration.
BagusJeep lives in Bali.
1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1951 Willys CJ3A/MB/M38 - Little Willy
1995 Cherokee 4.0 - CHEROKEE