The drop rates of both of these items are very hard to compare, because Aberroth has no external modifiers to drop rate or difficulty.
While Shade has multiple axis of scaling:
- Corruption
- Distance from the starting point
- %inc. Item Rarity
Especially the 3rd one with the %inc item rarity has gotten a lot less substantial since the latest changes that made modifiers last only 3 echoes maxium so you can only stack 2 modifiers onto the bosses.
But both corruption and distance from the starting point are both huuuge contributers to the chance of the non guaranteed drops (Apathy’s Maw, Shattered Chains and Omnis).
I do think that on deep Shades (12+ Corruption gain) the drop rate for Omnis is actually pretty much similar to Aberroth’s Nihilis.
But a lot people that chain farming Shades often fight the first Shades they encounter and thus reducing their own chances significantly.
Overall unlocking a 12+ Shade does take longer than farming a few Timeline bosses and drop a Harbigners Eye so Aberroth is significantly easier to access as well.
So I do think the drop rates are very similar, but accessability to the bosses is just very different.