The short answer is as long as your doctor or other provider accepts Medicare, they will accept any supplement you have. So regardless of your company you can be assured it will be welcomed by everyone.
The first reason for this is called “crossover.” Your doctor doesn’t actually bill the insurance company but instead sends it to Medicare. Medicare then pays their share of the allowable charges and forwards the information to the insurance company (called “crossover”). The insurance company then pays their share and does so virtually immediately. No company decides what is covered nor delays payments.
The second reason is federal law. No provider may suggest that one company is better than any other. And really, why would they when all plans of the same letter (ie, Plan F) pay exactly the same and just as quickly?
So if anyone tells you one company is better than another you might want to question if they have ulterior motive (such as an insurance agent) or just don’t know the truth (ie, your friends).