He's not that underrated. He's as good as the team in front of him, no better. He's probably the worst goalie to have won the Stanley Cup in the last 30 years.
And 300 wins isn't that impressive when you figure he's been in the league for 12 years, most of which were with Detroit. There are a LOT of goalies who could have won 300 games for Detroit in the 90s.