Here is just a list of some items I notice lacking the c:leather replacer compatibility.
Supplementary

• bellows

Minecraft

• Leather Horse Armor
• Leather Anvil repair recipes

Ars

• Annotated Codex
• Summon Steed Glyph

Irons Spellbooks

• Scarecrow Armor Set

Also, some compatibility inconsistencies I found.
Using Rich Soil gives less output than vanilla dirt. They should yield the same or more crop output.

Also, Torchberry from TWF is missing from the Cloche

The leather things I do not do replacement for other mods with to my knowledge, and if I do it has been inconsistent about working in the past. Those, you'll have to bring up to the respective mod authors about not supporting c:leathers considering it is an actual c: tag now. Leather horse armor I can work with, I don't think I can work with anvil repair.

Crops for rich soil that looks like the numbers I have in the code are either not applying correctly or the amounts changed on me, but I think the answer with that is that the counts I have aren't applying correctly. TF torchberries in the cloche I can also do.