We have eaten at Radiance restaurant on Koh Samui many times over the past 15 years, and we have seen it first rise and then fall.
First, the good:
The location right on the beach is nice. They make use of reclaimed wood, which is eco-friendly. Though Radiance is a meat-serving restaurant, they do also serve some vegan or veganizable dishes, several of which are tasty.
Now the bad:
1. As other reviewers have noted, the service can be terrible (slow, inattentive, rude, and sometimes even hostile). Of course not all are the same, but we've been shocked a number of times (e.g. even when our own server was OK, it was unpleasant to have to listen to a server at a nearby table shouting at their guests).
2. Prices are increasingly exorbitant.
3. Radiance routinely fails at special orders. For example, the menu says a certain dish can be made vegetarian/vegan, so you order it that way, but then it comes to your table with meat (or dairy or whatever you had asked them to leave out). So it is all too easy for a vegetarian to become unwittingly responsible for the death and suffering of animals. Making matters worse, Radiance staff are prone to argue with customers who complain about receiving something different to what they ordered.
4. Radiance management doesn't understand what "vegan" means (and refuses to be educated about it). A number of their dishes labeled "vegan" contain honey. On two occasions we brought this to their attention, but instead of correcting the error they argued with us, insisting that honey is vegan. We politely explained that "vegan" means "containing neither animal products nor products that have involved animals in the production or have been tested on animals," and we pointed out that honey is made by animals and then taken from animals (sometimes with little bits of bees still caught in the honey), but they dismissed us.
In short, Radiance Koh Samui is an overpriced meat-serving restaurant that does offer vegan and lacto-veg items on the menu but unfortunately can't be depended on to serve what the customer has actually ordered, and then can't be depended on to take responsibility to correct their mistakes — and won't necessarily be civil about it either.
For these reasons, we can't really recommend Radiance, but if you're in the area it's still likely better than most of the other meaty places nearby. Just be triple-clear about your order, to minimize mistakes.
Better yet, try one of Samui's new fully-veg places instead.
Cons: Too often serve the wrong items, service can be fair-to-terrible, increasingly overpriced