I'll consider flakes usable for packaging software when they support passing options. The respective issue [0] has been closed unfortunately. Perhaps I am misunderstanding what flakes are meant to be (a more formalized standard way to define nix packages and apps), but a lot packages in nixpkgs have a plethora of parameters that as of now can not really be mapped to a functionality in nix flakes.
It's nice that there is a workaround, but passing build options is not something that should require a library. There should be a well documented standard way to do it.
[0] https://github.com/NixOS/nix/issues/2861