I don't agree he is underrated. Actually the opposite in these boards, where you often see him mentioned as a top5MW of all-time, or even #2, which is nonsense. I mean, he probably is underrated by some noobs that just underrate everyone from a previous era but that's another story.
He was definitely one of the best in his weight range back in the day but he was once the top dog at 185 also because it's a fact that UFC wasnt willing to throw money to top talent in order to compete in that division, as they were in Rich Franklin, who was a great posterboy for them.
Dont you think that, apart from Hendo and Belfort, Machida was not perfectly able to make 185 and beat Rich, as he actually did beat him in the past? Machida didnt use to walk around heavier than Rich.
Don't you think Lindland's wrestling prowess would pose a serious threat to Rich in his prime?
Don't you think Paulo Filho would have been a terrible matchup back when Rich was defending his UFC tittle vs the likes of Quarry or Loiseau, who were getting paid per fight like not even half than what Filho was making in Pride?
Rich's fight with Okami was extremely close btw. A draw in that fight would have been a reasonable score.
As I said, a great from his era but I'd not say he is exactly underrated, more like the opposite, especially by American fans