Well, to say that this topic has been an interesting debate of such a subjective topic of the definition of "underrated" would be an understatement, to say the least!
To me, personally, I think of underrated cartridges as those that perform beyond their paper ballistic numbers in their performance and effectiveness on game in the field. Many cartridges can be deemed underrated, when compared to the newest whizzbang of the day, dependent upon the given point in time.
As found in the dictionary, it is to "underestimate the extent, value, or importance of (someone or something)".
Many have put forth the idea that today's time tested and proven cartridges, such as the 30-30, 270, 30-06, etc. are underrated. And perhaps they are.
It would be a matter of individual perspective.
As each of us have entered this past time at different points in time, whether it be during the smokeless powder era, Magnum craze, the Ultramag craze, the short, fat craze or the Creedmoor craze, we have been either subjected to the bigger, heavier school of thought of Elmer Keith, the lighter, faster school of thought of Jack O'Connor and Roy Weatherby, or the more efficient case design of the PPC/WSM school of thought, and so on.
I am almost certain that many shooters/hunters of bygone eras had the same types of thoughts and conversations that we are sharing today, when smokeless powders came into being, when the 6.5's became one of the most common international choices of militaries around the world before/during/after the World War I (please forgive me if some of my timelines are not historically accurate, I am not a historian, these are just illustrations of the point being presented), or in the various decades since as metallurgical, ballistic and technological advances came into being, and still continue to advance.
It would appear the one constant is change, over time. And that change appears to be the definition of what is now underrated today as compared to what it once was. Not the fact that the cycle is repeating itself in that we have newer cartridges for comparison to older ones.
It is an ongoing debate that will never cease as we each have our own favourites that we will champion against those of others. Great conversations to be had between friends enjoying time together around the hunting campfire!