Edmunds was drafted under Keith Butler's defense that was based on having ridiculous athletes at linebacker and safety. He has been a starter since his rookie season and has been incredibly durable, missing only three games and consistently being among the team leaders in snaps played.
His big strength is his versatility that his athletic ability affords him. He can fill in at linebacker against the run and can run with almost anyone in man coverage. He is by no means a shut down man cover guy, as he lacks the technique and doesn't have great ball skills, but he can run with them and cut down the window for the quarterback to make the throw.
Terrell Edmunds is a true "Jack of all trades, master of none" player, he doesn't generate much in terms of splash plays.
5 interceptions, 1 fumble recovery, 5 sacks and 15 tackles for a loss in 79 games played isn't going to impress anyone.
I call Edmunds a matchup canceller, because he can cover almost anyone in the NFL and limit the damage that player can do. (again, not shut them down, but stay with them enough to make the throw tougher) He would frequently step in and take over man coverage on a receiver when the offense orchestrated a mismatch with a top receiver on a linebacker. That worked really well for the Steelers as a complement to Minkah Fitzpatrick, as he could take on any role and let Minkah play to his strengths.
One of the best examples of how versatile he is and how many roles he filled for the Steelers was the Steelers/Dolphins game this year.
Here's an article (film included) from that game.
I'll miss Edmunds on the Steelers, because even if he wasn't a great player, he was one of the unsung guys that do the dirty work that let our stars excel.