I understand where you're coming from but they don't have feature parity. Flux allows me to target much lower color temperatures than MacOS' built in "Night Shift" feature. That alone makes Flux worthwhile.

Does it take a long time to get used to that? For me, Night Shift at full warm makes some websites difficult to use due to the change in color. I can't imagine going even warmer.

out of curiosity, how low are you going in terms of temperature?

It's nice that they are there for people who won't install 3rd party software to get the extra features, but f.lux is still better.

I would argue that if some software is under a free (libre) license, it's worth using over [non-free] first party stuff. This doesn't apply to f.lux, but does to redshift.