Seems like a top-tier guy who beat very, very good fighters but came up short against some of the true ATGs of his day. I know little about him except that he was a featherweight champ and that he fought in the Olympics and apparently had an extensive and international amateur career, something pretty rare for those days.
He was a very good fighter. One of the most muscular of the early fighters Ive ever seen, especially for being so small. Its very impressive that Armstrong was able to cut weight and take such a high quality scalp.
Probably underrated IMO - after all, he won a ton of fights, was only ever stopped once by one of the greatest of all time, and holds wins over HOFers Freddie Miller and Benny Bass. From what I can glean from reports, he was reputed to be a tough pressure fighter, inside technician, and volume puncher, though tended to "slap" or "cuff" with his punches. He was not regarded as a big puncher.
