He was one dirty bastard who covered up his dirtiness under the guise of "bad tackling technique". And did fuck all in Europe. Zidane, Pirlo, Xavi, et al are miles ahead in that regard. The overrating comes from the fact that he's the most un-traditional English midfielder since Charlton.
Mind you, he was the only United player I actually feared when Arsenal played them in the late 90s and early 2000s. Vieira struggled to cope with him more than he did Keane imo