Optimal twist rate is dependent on projectile bearing surface. The rifling does not care what weight the bullet is, only how much of it can be engraved. Longer bearing surfaces don't do well in loose twists.
With a lot of the target bullets, as the weight gets heavier, the bullet bearing surface also increases, but secant ogives and long boat tails cut into this length as well.
The next issue has already been mentioned, which is gyroscopic stability. Since many of us are interested in long-range performance, we want the projectile to spin faster at distance to maintain stability. There are charts and programs to compute your stability factor with a certain twist rate, bullet type, and barrel length.
There is a corresponding relationship between spin stability, twist rate, bearing surface, and barrel length, which is why DTA uses a tighter twist on the 16" covert. To get an equivalent bullet to spin well at 800-1200yds from a shorter barrel, you need a tighter twist because you're starting with slower speed.
A tighter twist then has implications for internal ballistics, because start pressure changes. Now that the twist is tighter, the projectile needs more force to drive it into the lands, generating more peak pressure compared to an equivalent charge with the same bullet in a looser twist.
There does seem to be merit with the shorter bull barrel and tighter twist, which Todd Hodnett has been getting unusual results with in both .308 16" barrels and .338 LM 20" barrels. If you follow what he's been doing, he has experienced better long-range accuracy with the short, tight twist bull barrels well past 1000yds.
So I actually do think there might be some merit to the experimentation with a 1/8 twist in the .308 16" with 175's. Todd was pushing to keep the bullet spinning consistently as it passes through the transonic barrier, and his results seem to indicate that he achieved that in spades, especially when a tight twist 20" .338 TRG is grouping better than a 29" at 1 mile. Very few shooters actually have the skill and experience to do any type of conclusive testing at those distances, but as long-range shooting becomes more popular, I think these numbers will grow.