2 part filler gets used a lot, loved because it dries in seconds. It's just unnecessarily hard to sand flat. Normal decorators filler does work but it's softer than wood filler.
Not just to sand flat. It takes an age with hand sanding, with a sander it just melts and clogs the paper. It's REALLY not good for you (carcinogen) it has been strongly linked to COPD in car body workers and trades that use it regularly.
I think I've a tub of that in the shed, the Yorkshire in me might resent spending much to hide 6 screw heads :)