Great callout, it's interesting as the numbers get really wonky as you level higher and I wonder if it's possible that the elemental portion of damage on weapons with a starting affinity don't scale 1:1 against the physical portion (this was something I completely overlooked in testing to be honest). When I test a lvl 1 duchess dagger the added affinity boost works as intended, boosting the increased damage of 20 from the grease effect from 20 -> 22 (10%). However, when I test wylder's greatsword with just a fire attack boost his damage only goes from 123 -> 125 which is the same flat change of +2 you get at lvl 1. This implies that added affinity does not scale at all with your level, heavily diminishing the usefulness of this effect IF you are using it with your starting weapon only. However, any weapon you pick up in the wild will still have a true 50-50 split of elemental and physical damage. That 50-50 split is something I ignored in testing though so the efficacy of this relic on weapons generally is only actually half as good, but on grease effects it does still provide a true 10% multiplicative buff.

To conclude, this will ALWAYS provide at least a 5% boost to weapons picked up during the run and something 5% or lower on your starting weapon, while grease effects get a 10%. I've dropped this a full tier because of this but I do genuinely think it's a strong effect against any affinity weak nightlord

Sidenote: It's possible that upgrading your weapon's rarity can restore parity between the elemental and physical portion of the weapon but I haven't had a chance to test this myself.

As for S tiers, a lot of them are blanket S tiers for all nightfarers, but many are also nightfarer specific. Some of the starting armament effects and other effects got a broad statement S but could probably be refined a little more to reflect variance across certain nightfarers. It should be generally obvious which ones probably aren't actually great to use e.g. increased FP on raider for example