Reading through this list, I was struck that I had acquired several as vintage bikes, but I never saw them when they were new, and/or I didn't own similar bikes at the same time. It's hard to say something's underrated when you don't really get a chance to compare! Most of the bikes I've come across as vintage were mid or low-end, so it's hard to say a Motobecane Mirage is underated vs. a Raleigh Gran Prix, but I think it has a plusher ride. I've had a Zebra, a very similar Nishiki, a Shogun, all with the popular "sport touring" longer wheelbase type geometry. They were ok, mid-level mediocre.
I had an Alan Super Record for a long time, I never had any structural problems that are attributed to them- but when I built up a Vitus 979 years later, I found it to be a much more comfortable ride and less twitchy. I never tried a top level steel race bike until I found my '84 Schwinn Peloton, it's a great bike and a fraction of the price of something Italian. I had a LeTour Luxe, Granville-made, very solid bike with SunTour Mountech, just too big for me. I used to make fun of Schwinn's, now I think good ones are underated.