I love custom NPCs, but its pretty buggy in 1.12. and yes I have reported the bug just not fixed yet.
But ignoring that, for me it's too easy to exploit. For example the merchant system. A player can spawn in an NPC and give them any trade they want (dirt to diamonds) and boom. Ive asked a config to be added for that but it hasn't yet (meaning a config to disable certain roles).
Hopefully, that mod author comes back and does some work ... but I would always love more options and more mods doing NPCS