Here is the only metric that matters: number of players who end up on the injury report due to contact, not talking about those non-contact ACLs. Bernard Pollard, Ronnie Lott, Steve Atwater, Mike Singletary, Ray Lewis, = violent.
As for the Steelers, I feel their reputation provides some bit of cover for them to make hits that many finesse teams aren’t allowed. To that end you need players who are fast and big. Remember Seattle’s Legion of Boom secondary? The broadcasters bought into that labeling, and as a result they were allowed to hit players more dangerously than many secondaries. This ultimately cost Cam Chancellor his career as he suffered a neck injury to the impact of many violent collisions.
Defenders known as “hitters” aren’t made of steel. Eventually they all breakdown, victims of their playing style. The best defensive units are therefore better measured by how tactical they are, and how good they are as measured by; yards against, and points against.