i've thought the same thing. but on the other hand if you go through his playoff record series by series what you see is he is very good at coming through against normal teams but (statistically anyway) disappears against the tougher competition.
still, he's put up some surprisingly strong performances for a guy with his reputation, even though he does also legitimately wear the disappears in tough games/series reputation.
here's his prime, where he put up 101 playoff points in 101 games—
as a pre-rookie in '88 (given that he joined the team after the olympics as a 20 year old this is a fantastic showing):
6 games, 5 points against buffalo
5 games, 0 points against montreal
7 games, 9 points against new jersey
5 games, 2 points (both assists, none in the final three games) against edmonton
1989:
5 games, 7 points against buffalo
5 games, 6 points against montreal (this is super impressive, actually)
1990:
4 games, 8 points against hartford
5 games, 6 points against montreal (four of them in one game)*
4 games, 8 points against washington
5 games, 0 points against edmonton
1991:
5 games, 8 points against hartford
7 games, 6 points against montreal (four of them in one game again)**
6 games, 8 points against pittsburgh (six in the first two games, which they won, two in the next four, which they lost)
1992:
6 games, 6 points against chicago (he's on st louis now)
1993:
4 games, 6 points against chicago
7 games, 5 points against toronto (all five points came in games 2 and 3, which they won)
1994:
4 games, 4 points against dallas
1995:
7 games, 7 points against calgary (on san jose)***
4 games, 0 points against detroit
and after that he's just a journeyman putting up a point every other game on also-ran jets/coyotes teams that never got out of the first round.
* that one game was phenomenal. i can't remember exactly but i think it was THE cam neely is in roy's head game. janney assists on three out of boston's four regulation goals, including neely's PP goal at the end of the third to send it to OT, then sets up galley's OT winner. boston wins its second straight one goal game to take what would turn out to be an insurmountable 2-0 lead.
** another classic neely owns roy game. janney assists on all four boston goals, hat trick by neely... BUT he also fights denis savard with the game out of hand late in the third.
*** this was a fantastic performance. five points in the last two games of the series to come back from down 3-2 to upset first place calgary. what a weird team—janney centering pat falloon and jamie baker, and the other line was young ray whitney with larionov and makarov.