CBD can have quite good effects on calming anxious dogs, relieving pain, and possibly for seizures.
But it can also slow heart rate to a dangerous level, especially for small dogs with large doses given. I have discussed this with my vet and she said she has seen some worrisome issues with small dogs but has not, as of now, seen issues with larger dogs. She agrees that it can help, but that a test dose should be used when an owner can observe and take for vet care immediately if the dog shows any signs of lethargy, breathing issues etc.
It is currently an unregulated supplement so the dosage from any given supplier is a trust based on their lab tests provided. There may also be legal issues (U.S law) depending on state and the form of the product.