We've had the Invisible fence brand underground fencing here since 1992, and it's worked well most of the time.
Our eighth dog, and adult knothead rescue, decided that getting to the chickens was worth it, in spite of serious efforts on our part, and the IF people.
At that point we put up a 'real' fence, 5' high, with the IF about 3' inside the physical barrier. Perfect!
Until about 5 years ago, when the control panel died and we didn't realize it. Then another rescue, a former fighting dog abuse pittie, dog under the Ft. Knox fence and killed many of our chickens in a short time. Awful!
When we figured out the problem with IF, they replaced the collars, batteries, and control panel, so everything works fine again.
When we raised our dogs from puppyhood, they learned to leave the chickens alone, and respected the IF. Adult rescues have been tougher, and now we depend on the real fence plus the IF, and keeping our birds out of the dog area.
Long story, and I think that properly managed IF collars can work very well to keep your dogs in. Little dogs especially are still vulnerable to attack from invaders!
I tell everyone with a small dog especially, to take pepper spray, at least, when walking their small pet.
Mary