> IIRC it still has problems when output goes across multiple lines > (becomes continuous underline including margin and spacing from > second line going forward) and with underlining punctuation after the > argument (workarounds force an extra space). I can confirm tomorrow.
The underlining from the continuation line left margin actually occurred with the basic line drawing macro from the manual that was mentioned here by Branden as ".Underline." Seems like a reasonable limitation of that approach being anchored to the start of the string and handling punctuation properly. The underlining of trailing punctuation was an issue with Werner's macro. Adding the punctuation as a second argument after the quoted underlined text produces an extra underlined space and punctuation mark. So .Underline "intended underline" ! produces _intended underline !_ rather than _intended underline_!