I never fully realized before, but generally speaking, veterancy just negates itself. Exceptions aside, vet0 squads will roughly perform like their vet3 equivalents against each other.
The most common RA buff is -29%, some squads like volks and penals get a little bit less than that with -23%.
Most common veterancy is +30-40% accuracy, together with -20-25% of cool down.
This means, that the RA buff of -29% is negated by +40% accuracy, the -23% by +30% accuracy for the opponent. They cancel each other out, what is left over is the cool down, but the effect here is usually not THAT great.
So what you say for the Ranger/Grenadier matchup is pretty much on point, there is only a marginal DPS increase (I calculated a +4,5% DPS increase due to the 'leftover' cool down buff). But is also works vice versa: Grens get +40% accuracy, which is negated by the -29% RA for rangers.
Where Rangers seem to fall flat is when they match other squads. The 20% damage reduction is the only defensive buff for Grens in the whole game. Vs small arms, it is one of the worst buffs of any squad, offset by the effectiveness against AoE damage.
But against many other squads, e.g. Obersoldaten, Rangers will actually lose DPS (about 7% against Obers and all other squads getting -29% RA), while those squads will still get at least the marginal DPS increase due to CD. I think that is what people are noticing when they say Rangers were not that good in the late game.