He was arguably the most all-around dominant forward in the league before the head injuries..
I am fully aware of that but at the end of the day it is what you accomplish and for a guy of his stature he should have been a shoe-in for the hall of fame i.e. like Pronger. Likewise, I think he has been unfairly treated by the hall because of his feud with Bobby Clarke, refusal to report to the Nordiques and etc. (all nonsense)
I'll use an example from my point of view using Connor McDavid, if McDavid never reaches the playoffs with EDM his career will be viewed as an overall failure for a guy who was so highly touted. Hockey is a team sport but hall of fame players should be able to will their team to victory at some point. Obviously the Flyers and Edmonton were/are totally different teams. The flyers were suppose to be contenders while Edmonton is a bottom dweller.
At the start of Lindros' career did anyone expect him to never win a Stanley Cup? I'd be willing to wager most thought he would at least have one ring if not multiple. Failing to meet expectations is definitely what I would consider as underachieving. Did anyone expect Crosby and his Penguins to have only won one cup by now?