Most of companies are small businesses which are unscrupulous and rent cars insured with the minimum legal coverage for a private vehicle. Most of the time their cars are not insured to be rented. You should also know that quite a few Thai insurance companies do not cover foreigners unless they have a Thai driving license.

So if you rent from such companies ask at least that they write on the contract (yours and their copy) that the car is fully insured. And even so you will never be sure that the insurance will cover you. So better rent from a well established company. It is maybe a little more expensive but at least you can sleep well... and if something bad occurs you will be well protected.