Niedermayer has become overrated. In his first decade in the NHL, he had one great season (1998). He only played at his peak level from the 2003 playoffs until 2007 - but since that was pretty late in his career, many people project backwards and assume he had been at that level his entire career.
During his first decade in the NHL, he got less consideration for the Norris trophy than Eric Desjardins and Sandis Ozolinsh, and only slightly more than Teppo Numminen. Not that Norris voting is a perfect measure, but in this case I think it accurately shows that he didn't become an elite performer until very late into his twenties.
He is over rated in certain circles but even though he had only 1 great season in his first decade he was still very very good.
At age 21 he had an excellent SC playoff run with NJ and is arguably a 1B to Stevens 1A that year.
At age 23 he was good enough to play for team Canada in a best on best tournament.
At the age of 29 he leads the NHL playoffs in socring and plays 26:07 in that run.
In the previous 4 years he averages 25 MPG in the playoffs over 56 games and 2 trips to the SC finals.
Part of the problem for Nieds is that he played in Scott Stevens shadow.
I still take Keith in his prime but I think Nieds had a better career even if only slightly.