I recently flew KUL-USM-KUL, the flight to Koh Samui was almost empty, but they still flew it. The flight back was full. Not a problem. The plane's are older than dirt, but got me from point A to point B safely. USM is one of their hubs, so even if there's a issue getting you to BKK, they can route you through another airport like Phuket. I think you'll be fine. When I flew USM-KUL, I was connecting on to an MU flight to PVG that departed at 1:50am, by Bangkok flight was the last/only flight to KUL and I wasn't worried. Granted, I think this is a matter of your tolerance for risk. Mine was quite high. Worst case, you miss your flight home. Then what? Do you have the stomach to fight and get the airline to get you back to the US? Or, more likely, the weather in Koh Samui is so good you only need to worry about a mechanical delay or bad weather in BKK, which is unlikely.