These are accepted at LHR on Flights departing LHR and BOS BA215 and BA214 the sunflower lanyard is accepted. However the sunflower lanyard is not accepted at BOS to be allowed in a medical certificate will be required. Getting around America is difficult as business tend to discriminate and say if you can’t wear a mask you can’t come in. And all public transit is like that so for example if you want to get from BOS to LAX you have to connect in LHR absolutely ridiculous. So the answer is US would not be a good destination for someone who couldn’t wear a mask or couldn’t wear for extended periods of time or found it unpleasant or difficult. My advice is travel somewhere where your exemption is accepted. I have done this myself as I am exempt from wearing a mask so I know how the process works.