If your 8-year-old has confirmed fluid behind the ears and reduced hearing as confirmed by audiometry and tympanometry (type B curves), and adenoids blocking more than 50% of the nasopharynx, as seen on x-ray, and this has persisted for more than 3 months then he needs gromets, with or without adenoids (as per results as mentioned above).
Surgery for gromets is totally safe. The only slight possible complication after the surgery is of minor ear infections. Hearing will improve. Adenoids surgery mean that there is a minimal risk of bleeding. And the child needs to stay in overnight. Any general anaesthesia or surgery carry a risk of complication, the chances of which are similar to your getting hit by a truck when crossing the road.