Here’s a half-baked thought experiment:
Poisons scale exponentially for sure, but they drop off after 4 seconds. So poisons take 4 seconds to reach maximum damage, but that maximum damage is higher due to each poison adding ‘increased damage per stack’.
Bleeds scale a bit differently, as they have no ‘increased damage per stack’, but they do have armour shred applied for 4 seconds on each hit. (I have no idea really about armour shred: it seemed to be pretty weak in earlier patches, but may be better now if monsters have a new ratio of protections to life?)
So with 136% bleed duration, we expect bleeds to last about 9 seconds, meaning they take 9 seconds to reach maximum damage. The first 4 seconds get exponential growth from new bleeds as well as new armour shred. The last 5 seconds just get new bleeds.
(important question for DPS -If a minion hits a bone cursed enemy, does that count as player applying the bleed or minion applying the bleed?)
In gameplay, both poison and bleed have an extra factor for damage, which is the number of monsters your minions are attacking. Hoping that the number of enemies is decreasing (we are killing some), this would mean hits on a single target later in a fight increase over time. This may act as an environmental source of ‘ramping’.
So it seems bleeds take longer to ‘ramp up’ to max damage than poisons, which might mean that they get even less benefit from brittle bones.
Alternately, any time saved attacking an enemy with fully stacked bleeds will allow more time to start stacking bleed/armour shred on the next enemy, so maybe it’s not an 18% DPS increase on a single enemy, but it’s roughly an 18% increase on a fight vs many enemies?
on second thought - Any boss fight that lasts longer than 9 seconds would get full value from brittle bones, so it is fully effective where it counts!