We stayed at Sunset Beach in September 2019, the so-called "Rainy Season". Our stay was absolutely perfect.
First and foremost, it is on the quiet side of the island and away from all noise. If you are considering this over Chewang Beach, do not hesitate. This is a lush getaway - the cities are crowded parties.
We were greeted with coconuts and we practically had the place to ourselves due to the down season. The property is immaculate, the service is perfect, the beach is nice and the water is warm. They arranged for a schooner to pick us up and take us snorkeling, as well as a scooter rental so we could zip around. It was our own personal Corona commercial - I wouldn't change a thing. No bugs at all, rooms clean, in a word - paradise. This is definitely a hidden treasure on Koh Samui.
Only one single complaint: the food is too expensive. There is a very good beachside cafe about a 5 minute walk down the beach that is literally half the price of the resort restaurant. They were also sensitive to us dining off the property because it was the slow season. But the fact of the matter is that the food options nearby are just too cheap to ignore, and the beach cafe is every bit as picturesque and secluded as the resort. This is the single complaint.
Do not hesitate to book here. You will not regret it. It will be your own personal paradise.
Oh yeah and the massage was the best we had in the country.