My sisters TB is fired and is sound as a pound, she has BEAUTIFUL paces and is only lame if she's had a hooly in the field and has kicked herself - the tendons never cause problems. I also used to ride another horse who was fired and again never had a problem.
Pinfiring is a method used on horses where the skin is burnt so scar tissue is formed - when scar tissue forms it shrinks so basically forms a permanent tendon boot on the legs, it's typically done when a horse has had a fall and there is a small tear or hole in the tendon. Although I have heard of this being done as a preventative method as well.
It depends on what you are lookin to do with the horse and how you feel about them already, if they are otherwise perfect for you and you don't want do 5 star 3 day events then all I would sat is when you have a vetting done ask the vet to takke a look to see if there is any obvious tendon damage that can be seen?
I know many horses that have had this sadly done but have gone on to be fine
think there is a campaign to make this process illegal due to cruelty