What is the controversy around their method?
It's very simple, really. Simple, but incredibly hard.
The Dhamma retreats vary from country to country. I'd suggest you pick one 1) in a nice, tranquil environment, with 2) good, healthy food.
I did mine in my home country (The Netherlands) and it lacked the latter. It's not that the cooks didn't try but PB&Js on stale bread for breakfast is hardly conducive to a good practice. Compare that to Thailand, where you'd get served delicious local Thai cuisine.
I'm warning you though, it will be a very, very intense experience. This shouldn't discourage you, just don't approach it as a nice, relaxing vacation. Meditation is hard work. Give me shoveling in the dirt, all day.
P.S. Make sure you adequately protect yourself from mosquitos and other nasties. I had to leave mine early because of a severe infection after being bitten by something fierce that could only have emanated somewhere deep from the throughs of hell.