I'm not sure what either classic rock or underrated means anymore, but I don't think any of the ones they mentioned were given short shrift by anyone but critics and journalists. However, I'd like to add some others...
Three Dog Night - The classic lineup was not to be trifled with, musically, vocally, or in terms of gold records yet they are all but ignored in these types of discussions.
Grand Funk Railroad - Loved by fans, hated by critics. Mark Farner is a f'ing legend.
Uriah Heep - Calling them "the poor man's Deep Purple" does them such a disservice. The live album is a tour de force and should be required listening for all bassists.
J. Geils Band - Overshadowed by their 80s product, but an incomparable live rock band.
Foghat - Nobody boogied better. Should have been American.
My two cents (and I like all those bands):
Three Dog Night: They have been - regrettably so - consigned to oblivion because they never wrote their own songs, all their hits were (very good) covers. I wrote about this a while back: That inevitably costs you recognition with longevity.
Grand Funk Railroad: I believe rock critics were hung up on The Jimi Hendrix Experience and Cream as being the original and therefore only worthwhile guitar-led trios of the late 60ies, all others that followed (West, Bruce & Laing too) were viewed as pale imitators. No, Mark Farner was neither Jimi Hendrix nor Eric Clapton indeed, but GFR were an extremely organic outfit with a timeless groove - that aspect fell by the wayside as critics judged them. If Led Zep kicks you off a tour for going down too well, you must be doing something right. Irrespective of the music, Terry Knight's management of the band rubbed a lot of critics the wrong way too.
Uriah Heep: With that overly loud organ and a tenor/falsetto singer they did indeed follow the Deep Purple blueprint quite a bit (both bands shared adjacent rehearsal spaces for a few months in 1969, legend has it that the Purple guys would sometimes come back from the pub only to hear Uriah Heep jam on something they had been working on before
). For a time (1971-73), Heep were huge in Germany, bigger than Led Zep or Sabbath, but not quite as popular as Purple. They lacked that virtuoso image Jon Lord & Ritchie Blackmore had (Ken Hensley was a self-taught piano and organ player as well as a guitarist, Mick Box essentially a rhythm guitarist) and didn't excessively celebrate that jammy improvisational frenzy of DP live, but their mostly Hensley-penned songs were catchier, they had more singles in the charts and the prominent harmony vocals (said to have left a lasting imprint on early Queen) gave them a radio-friendly sheen mostly missing with DP. I think they suffered by putting out too much product too quick - six studio albums within the first three years of their existence (+ the Live album)! Hensley was creative, yes, but not THAT creative. Their escapist, often fantasy lyrics did them no favors with critics either, they were not seen as sociologically relevant spokesmen of their time.
J. Geils Band: I'm extremely partial to them! Among my top ten live rock experiences ever was a gig of theirs in a club in Wiesbaden around the Love Stinks album. I thought Peter Wolf was one of the great American frontmen, as good as Steven Tyler, but a lot cooler. Cooler than Jagger too (there were a lot of comparisons, especially when the J. Geils Band opened for the Stones on their Still Life World Tour). From what was originally a white RnB covers band, Seth Justman and Peter Wolf really evolved into a good songwriting team. I even liked Freeze-Frame (the album); too bad that they split up at the peak of their commercial success, Wolf should have endured Justman's blossoming pop leanings a while longer. Criminal that they haven't been inducted into the RRHoF yet.
Foghat: They single-handedly prevented the success of Status Quo in the US of A!
You already had Foghat (which conversely meant nothing in either the UK or Continental Europe).
But I'm afraid that (i) being heroes of the Midwest rather than an established East Coast or West Coast draw plus (i) a not very pronounced sociological message in their lyrics either did nothing to endear them to the critics caste.