In my experience it's rarely the best of both worlds, it's more of a case where the IDE becomes more tolerable.
The problem comes in when the IDE struggles to keep up, or is busy trying to move my cursor somewhere that I don't want it to be.
IntelliJ is really bad about this, I'll often type something and hit esc to go back to normal mode. Mentally I'm moving to another line using a Vim motion, but IntelliJ lags, and eventually moves the cursor to some problem it thinks I need to solve right now (and I might or might not be back in insert mode).
It's frustrating, but probably less frustrating than not using the Vim bindings. Maybe it's something with the Vim plugin, or something I've done. It's more of a minor headache as opposed to a migraine.
AUR is a supplementary set of packages, and it looks like you're comparing it to the total number that Nix supports.
Wouldn't a more fair comparison would be official Arch packages + AUR to nixpkgs?