What are the grounds?
Getzlaf..
Top ten point producer in the league 3x
Top ten assist producer in the league 7x
Hart finalist
Played above Bergeron in international competition.
Was the actual # 1 C on his cup winning team
Bergeron..
Won Selkes.
Was good as an overager in a WJC.
What is the “on ice impact”?
He was never a top ten producer in the league. He was never acknowledged for anything but defensive play by a bunch of media guys who typically just sort by face off %.
On-ice impact is the ability to control play and create goal differential. This is done through offense and defense, maintaining and advantage in possession, shots, chances and ultimately goals through both creation and suppression - not just individually but for all the players on the ice.
The best measures are xGF% and GAR.
Individual production is obviously one way to impact goal-differential, but it is far from the only way. Exceptional producers can have great on-ice impact (like Crosby) or very weak on-ice impact (like Patrick Kane). Exceptional defense (suppression), possession, chance creation for the team can ensure high goal differential even if individual production is lower.
I cant speak to the motivation for any writers to rank Bergeron highly, but his defensive impact certainly extends beyond faceoffs. His shot suppression and shot generation are both elite. He very simply always has the puck… and since you need the puck to shoot and score, that makes him very effective defensively.