Would depend on the size of island you start on I would think. Something decent enough for 3 to 4 cities should carry you to shipbuilding quickly enough, get a small continent with rivers and you're set. They would do very poorly on a smaller island tho especially with no rivers.
I feel like the Khmer would have a tougher time finding ideal spots overall, remember you want at least 1 fresh water space for the buffed aquedect and preferably a river tile in your second ring for a holy site, assuming you want to exploit their unique relic ability. That amount of contiguous river will surely be more rare on island plates vs. Continents.
I am pretty sure reliquaries doubles yield from relics of both faith and tourism, meaning you'll get a crap ton of faith out of them even if you don't go for culture Victory, would make for a very strong religious play.
I played a shuffle game with them I just finished, dragged out to turn 576 (epic) because I had to kill teddy to get the culture win. If you're th8nking of going the culture route that would be key advice, don't neglect culture districts. Yes your relics are nice but those holy sites won't fend off another players tourism.
I had my cities grow to a much larger size than they would be normally in my first game with them tho by the end of the game that had levelled off, so I would definitely prioritize science assignments as you get that additional population if going for science. I was very much behind on science in my game.
