Absolutely he's underrated.
A lot of it is because he 'plods' along at 4.something yards per carry and just moves the chains. He's not an explosive, SportsCentre type back.
His longevity and productivity is ridiculous - he's a first ballot Hall of Famer, but he's not spoken about as one.
You said what I wanted to say, but better. Here's a good example; Frank has averaged 4.4 YPC for his career, while Le'veon Bell has averaged 4.3 YPC, over a smaller sample size, but gets a lot more publicity as one of the best backs in football. Now, obviously Le'veon has better numbers catching the ball, but you see my point