As far as I was aware AMDGPU is used by default on most if not all distros

I really don’t think that’s the case, assuming you’re talking about AMDVLK (amdgpu is the kernel module used by all three Vulkan drivers - RADV, AMDVLK and the Vulkan driver from AMDGPU-PRO). Ubuntu and Fedora definitely default to RADV, and Arch Wiki recommends RADV unless you need something from the other drivers.

I noticed a performance increase after forcing RADV on NixOS so not really sure.

NixOS seems to default to RADV according to their Wiki. If this was a few years ago then maybe you might be confusing it with the ACO shader compiler for RADV? That brought a significant performance increase and eventually became the default in RADV. I remember using custom Mesa (the project that develops open source graphics drivers, like RADV and radeonsi) builds to massively reduce stuttering in DirectX games.

I personally chose RADV after looking into this myself and the only drawback from my understanding is that they are proprietary drivers.

RADV is the open-source community developed Vulkan driver. It has the widest hardware support of the three Vulkan drivers and is generally the best for gaming.

AMD provides two more Vulkan drivers - AMDVLK is the open-source one available in AMDGPU, then there’s the unnamed proprietary Vulkan driver in AMDGPU-PRO. The biggest advantage of the proprietary one is that it is certified - doesn’t matter most of the time, but when it does, a missing certification is a deal breaker.