HD is basically like this:
In an area where there is a first adjacent on one side, might have problems
First adjacent on both sides, limited if any signal. DX will cause this problem, as will translators.
Initially, HD Radio was licensed for about 1% of total power. Eventually they were given the go to boost to 5% of analog power. Stations that did not boost power from 1% to 5% will not have as good of an HD signal as the ones that did. Some of them are using older equipment. They're trying to get 10% but that will cause interference. It already causes a lot of interference on adjacents with 5%.
Basically they only care about the city grade area. Outside of that, the FCC will let interference run wild, and they don't care about HD reception outside of the city grade area. Boosting power to 5% has improved HD reception outside of the city grade areas, but it has also added interference to adjacents in neighboring areas. They can't do anything about that interference because the FCC no longer enforces interference outside of the city grade area. They used to force translators to shut down even if they interfered in fringe areas. 10% could really improve HD coverage in fringe areas based on what I've noticed from the 5% power bump, but it will also make a mess of adjacent signals. Some stations I believe will also have to upgrade to bump to 10% power. Only the ones who want to drop the money on it will do it. Audacy is already interested supposedly. New York will likely see 10% power at some point