There are many good and not so good ways to shoot off a pack - not to mention what’s in the pack and how it’s stuffed together. A springy light weight roundish or lumpy pack is the worst - it moves with the slightest movement and you’d be better off resting your gun on an empty one gallon milk jug.
1/2 MOA prone is doable. Shooting from sitting position, the pack upright is much better than walking sticks or a tall tripod, at least for me, and I normally cant do better than 1-1/2 MOA consistently. Once upon a time I tried a tall tripod while antelope hunting - the time to set up and jack around with getting the gun reasonably level was so ridiculous I never used one again after that.
At least with mountain hunting, 15 or even 5 seconds can be the difference between getting a shot off or not. Out to 500 yards if you can’t hit it quickly resting on a pack, the extra time to mess with the bipod, checking your ballistic calculator and dialing the scope will eventually cost you an opportunity.