The ZoL code is not the shoehorning, it's the fact that you're doing so via a community effort to go around the official stance of the kernel developers who've explicitly opted to not support this FS at this time; meaning you're missing out on all the normal development and QA processes and the benefits thereof. There's real value in having the the actual OS community designing the filesystem as well, their experience cannot be discounted.
This isn't just applicable to Linux; we have a strict policy on using only in-kernel filesystems on FreeBSD as well after some poor experiences (_in production_) with aufs-esque ports to FreeBSD. They all look shiny and nice on the outside, and even stress test OK, but when you release them to the masses that's when the shit hits the fan and you realize they're just not designed to the same specifications as the rest of the OS components.
This isn't just applicable to Linux; we have a strict policy on using only in-kernel filesystems on FreeBSD as well after some poor experiences (_in production_) with aufs-esque ports to FreeBSD. They all look shiny and nice on the outside, and even stress test OK, but when you release them to the masses that's when the shit hits the fan and you realize they're just not designed to the same specifications as the rest of the OS components.