Overall medical service in Thailand is pretty high. Technical equipment of almost all hospitals is on the very high standard comparing with the West. Detailed analyzes, MRI etc. are normal in Thailand.
But you should keep in mind that the way you going to be treated with tourist insurance is different. Tourist insurance has one specific focus: to provide emergence service, stabilize you and provide guaranteed departure to your homeland for a further treatment. Therefore nobody will try to cure your chronic diseases.
If something happens that need medical treatment, call your insurance company first and listen carefully what hospital you should go. In case if you need to be treated urgently and insurance company does not confirm, pay out the medical costs from your pocket and take the bills from the hospital. After confirmation you would be able to take money back from hospital. In case you’re living Thailand soon without insurance company confirmation, only way to return your money back is head up to the insurance company office, show them hospital bills and ask for a money-back.
Is it expensive? For example fever treatment would cost 600 - 1,200 baht, which means that you would be given medicine directly in the hospital and additional pack with pills to take it at your place until you feel better. All the instructions of how and when to take the pills, you would be given by your doctor.
Thailand is probably the best place for a plastic surgery in the world, because of great price/quality ratio.
Dentistry is also on a very high level. Simple filling would cost 300-600 baht. Canal filling 2000-3000 baht.