The answer is it depends. And if someone tells you otherwise, smoke what they are smoking.
Some folks will claim they can field judge an animal to X number of miles (plural) with hand-held 8x binoculars 100% of the time. That has not been my experience despite having alpha glass and extensive time in the field every year glassing.
I've seen Coues deer miles away with my SLC 15s. But then there's times I've lost Coues at 300 yards with my SLC 15s (deer bedded). So are my 15s on Coues good for "miles" or "300 yards"? I've glassed up mature mule deer bucks 2-3 miles away with my 15s. But I have also had issues determine gender on a desert mule deer buck under 800 yards (bedded tight under a bush) with the same 15s. So are my 15s on desert mule deer good for "miles" or "800 yards"? That same 800 yard mule deer provided the same results with a STX 95 (couldn't tell until it turned its head -- buck). Does that mean the STX 95 is only good for under 800 yards?
Do be aware that the further you want to glass, you need: 1) better glass clarity, 2) less vegetation, 3) more compliant animals (broadside in the the open and not moving), 4) better light, 5) larger animals, and 6) more magnification. The opposite is true; the crappier the glass, the thicker the vegetation, throw in a quartering away and/or obscured animal, the smaller the animal, and the less magnification means you cannot glass as far.
This is what I use in general:
* Desert: SLC 15s and my STX (65 or 95 depending on my goal). This applies to Coues whitetail, desert mule deer, javelina, etc.
* Mountains: SLC 10s and a spotter (Kowa 554 or STX 65 depending on my goal). This applies to Coues whitetail, mule deer, black bear, elk, mountain goat, sheep, etc.
* Misc: Mix and match from my optics arsenal for what makes the most sense. There are times I'll have SLC 15s and Kowa 554 to hunt bears in transition zones.