I have heard complaints about soft structured carriers “digging” under the armpit areas. On the Beco Butterfly, this is an easy fix! Instead of letting your shoulder strap cup around your shoulder and go directly under your arm to attach onto the front panel, try this:
Let the shoulder strap extend a bit farther through the buckle
Adjust the strap to go around your shoulder - towards your shoulder blade
The shoulder strap should then go down at an angle
It will connect to the front panel below your armpit, but not be able to cut in as they strap is extended that much further

Need a visual? That always helps me!

The strap on the left has been adjusted to come down my back while the strap on the right is cutting closely under my armpit.
You can see the strap on the left bending towards the middle of my back while the strap on the right is vertical.
The strap on the left is much more supportive as it spreads the weight of my toddler over more of my back and shoulder area.