fwiw, I have Kuiu (Ultra 6000, Icon 3200 and 185 packs), Outdoorsmans (Optics and Long Range packs), and Eberlestock (J107M) packs. My hunting style is public land spot-and-stalk, so I always have to be prepared to pack my animal out myself, usually solo.
In my experience, the Eberlestock provides the best carrying system I've used, by far. Gun is well-protected, and yet I can access the rifle quickly and easily - but I virtually never need to do that in my spot-and-stalk public land hunting.
(The J107M has a wider scabbard than regular J107, and Cordura front panel, so heavier.) Scabbard accommodates large scopes, with tactical knobs.
I've carried a number of animals (including smaller deer and pronghorn, whole, after field dressing) in/on the Eberlestock.
I also like the fact that you can still carry your rifle safely/easily in the Eberlestock even if you have meat in the pack or an animal strapped onto the frame.
The scabbard also works great for carrying a tripod and spotting scope if you're a spotter instead of the shooter.
However, for overnight or multiday backpack hunting, I no longer use my Eberlestock, because it is quite a bit heavier (almost 10 lbs. empty for the J107M), than my other packs (4-7 lbs.) and it has less room and weight-carrying capacity than my other packs. The frame is relatively short, which is ok for me, because i'm short (5'7"), but I expect the frame might be quite short for taller folks (thereby lessening the effectiveness of the shoulder stabilizer straps and the ability to shift weight. note that that is presumably why Mystery Ranch sells their load extenders for their frames.)
The Outdoorsmans system works well so far, based on limited usage. I'll be using the Outdoorsmans on a desert sheep in the next few weeks, and that will provide a better testing ground. Also, the Outdoorsmans' frame posts let you hang your rifle via the sling on the frame while hiking, which I have always found to be quite handy and comfortable. (Same as with using a Cabela's Alaskan outfitter pack frame without the top cross bar.)
The Kuiu rifle carrying system works relatively well, in my experience thus far. However, my buttstock barely fits in the carrier, and I do not have an unusually large buttstock on my rifle. And I personally can't get the rifle off the pack without taking the pack off - can't reach the release buckle and hard to extricate the buttstock from bottom carrier while wearing the pack.
And for me, I have to carry the rifle with the scope away from me (so my elbow doesn't hit the scope), and I have worried about the scope bearing the brunt of any impact if I ever fell/slipped on that (right) side. (I used the pack and rifle carrier in the Brooks Range, without mishap).
I have not used the Mystery Ranch carrying system.
good luck with your search.
