It also leads to everyone writing their own API addons that duplicate efforts and makes things incompatible with each other because everyone creates a patch that does what they need and only what they need. It also means that when Minecraft updates if you have a mod that relies on one of these additional specialized APIs, you have to wait on that author to update it, even though Fabric's main API is already current.
Neither Waila nor JEI require additional source patches to work with Forge. They have their own APIs, yes, but that's not what I'm referring to when I say that Fabric requires modders create additional hooks inside vanilla code.
Forge makes sure that every mod stays well out of vanilla code so that mods don't conflict with each other and crash the game.
I'm referring to this:
https://fabricmc.net/wiki/start#mixins_asm
Fabric pushes modders towards ASM. ASM is dangerous, complex, and highly fragile. It can lead to code that crashes code that isn't your code with the resulting stack trace having no evidence as to which mod caused the issue! It can DO anything, but it doesn't do it safely.