Great? He was a serviceable bottom pairing defensive defensemen, a bit of a stretch on the 2nd pair, but during his prime could hold his own there. He's in a class with Mike Commodore, Andreas Lilja, Hal Gill. An overall success story given his where he was drafted, absolutely. But nobody would consider him "great" by any stretch of the imagination.
He was paid way too much for way too long for what he was. That contract is an indictment on the failures of Ken Holland and Red Wings management during the late 2000s until the late 2010s. Ericsson was the best of a crop of drafted defensemen that has included Kyle Quincey, Brendan Smith, Jakub Kindl, Ryan Sproul, Hickets, Marchenko, and Nick Jensen.
That failure to draft any impact defensemen for 10 years is why the Wings have been a bottom feeder for going on 4 years. If Ericsson is your best drafted defensemen in 10 years, you're going to have a really bad time. And here we are.
Those failings are totally on the Red Wings. Ultimately Ericsson had a tremendous career for what he was. I wish him in the best in his likely retirement.