Barrie has been really struggling with his play on the number one unit in my opinion over the first part of the season. It may be the Girard is on the first unit due to his ability to gain the offensive zone with puck control (either by skating it in or by stretch pass), which the Avs have struggled with unless MacKinnon has the puck.
I think the first unit struggles on the PP relate to some structural issues (teams are attacking MacKinnon and Rantanen along the half-wall) and I am not seeing adjustments to account for that thus far.