> Well, one potential issue that springs to mind is that DC 120V might overstress the diodes in the full bridge rectifier.

I see what you're saying but it's a bit theoretical. Nothing in a switcher is run at that tight a tolerance, because in normal operation the load will be highly variable. One of the el-cheapo switching PSUs I have on my bench right now is rated for 90V to 270V, for example, and will run quite happily fully loaded across that full range (and considerably beyond - my variac goes from 0 to about 300V output).

> they usually have some circuits that use AC directly

AC motors won't enjoy it, but heating elements won't be affected.

If you look at the switching power supplies used for network equipment that often actually *does* have ratings for DC operation.