I was thinking the same thing. The biggest reason Rupert was so underrated in the series is in large part because of how Steve Kloves ( blame also does go to the directors and producers as well) wrote Ron. How many times were Ron's lines from the books and given to other characters? Scenes were cut out that really would've shown Rupert's wide range ( The "I love her like a sister" scene after the locket horcrux is destroyed is a prime example or Ron's sheer terror and hysteria when Hermione is being tortured by Bellatrix).

Rupert is phenomenal and was phenomenal in the Potter series but that's DESPITE Steve Kloves's writing and more because Rupert had such a great understanding of his character and because of JK Rowling's writing.