Yes, in my opinion the Candi is enough to make them a workable civ. It can generate more faith in one city than any wonder and can be built in all cities. In a recent Indonesia Piety game I was pulling 120 fpt in Medieval, I settled like six Great Prophets and looked like France. Plus it's a garden that can be built in any city - which fixes the Great Person deficit that coastal-focused civs often run into.
But their UA is frustrating even on island maps. I've floundered both times I've tried them on island maps, economy etc take too long to get up waiting for Compass and you don't really have any time to focus on religion.
The devs seemed to design Indonesia to answer the question, "what can be done to make settling off-continent more rewarding?" But a little free happiness doesn't fix the huge pressures on city-expansion syncing that cause most starts to fall apart if cities 2-4 aren't settled before turn 60 and if extra cities after then aren't settled ten turns after NC (because outside of that timing, building orders will constantly be out of sync and make National Wonders impossible).
