If your base is empty (no combat personal) UFO will destroy it instantly.
Defences reduce the number of enemies present, you can stack them + gravity shield so everything fires twice.
If you have sufficient firepower, its possible to destroy the UFO outright and you will be spared.
Though destroying large UFO will take some firepower, but for example, if Syndicate tries to enter your base via Scarab dropship (?) it will get pulverized and explode to thousand pieces before it lands.