Pierce had a very nice career, but he made just 1 2nd Team All NBA and 3 3rd Team All NBA. He never finished in the top 10 in MVP voting. Never made an All Defense Team.
He played a very long time and has great totals, but he never had a truly great single season and by the time he found truly great team success was past him prime.
I just have a hard time calling him underrated. He had a great career, will be a 1st ballot HOFer, but let's not pretend Pierce was more than he was. He wasn't an upper echelon player. Not like James, Durant, etc.