Don't take this as being smart Haji, but on reflection of what you said, of the fifteen positions which ones aren't critical?
There is a perception that some spots are more vital than others, which I am at times guilty of implying too, but really if any single position on a Union team is weak (at least at Pro level) then it is going to be exploited.
The style of match plan may cover some weaknesses on attack, but when defending you are going to get found out.
Interested in what others percieve to be the "easier" positions.
Certainly the Flyhalf normally is portrayed as a critical position, but if he has a crap Scrumhalf or is passing to monkeys then it doesn't matter how good he is.
Sometimes there is a suggestion of "easing" a player into a high level at Fullback, but that is one of the most critical positions in defence, especially in the modern kicking style of Rugby.
Loosehead is seen as "easier" than Tighthead, but then you are scrummaging against a Tighthead.
If you are carrying a Lock then the opposition will have a field day in the lineout and adjust their match plan to suit.
A great Openside can keep you in the match but not even George can get to every breakdown, so if the Blindside or #8 isn't mobile then it's going to be curtains etc etc...
I used to think there were "easy" or "less critical" positions on the paddock, but I'm not so sure anymore, at least not at S14 and Test level.