My experience was with a Carmax warranty that was supposed to be good anywhere.
When my mechanic tried to collect they would only pay flat rate, say 30 minutes for X job. He did not like it because they did not pay enough per job.
The dealer told me that when those programs started, they tried to work with them, but quit because of the above reason. So they wouldn't take it either.
Now it's GM or nothing for me
MD