Particularly for 40k, it makes sense to paint a batch at once. Probably a squad, squadron or formation.
It can start to feel like a drag, and standards may slip. Modelling and painting an individual mini is a way to experiment with new time-consuming techniques without it turning into a hassle.
Here's a one. After a particularly brutal defeat (using my Iron Knights against my friend's Death Guard), I rewarded my opponent with an Iron Knight zombie for his horde. I did not have any spare legs, but then zombies don't need legs.
There's greenstuff guts and mushrooms, UHU mixed with paint to give glue-string gore, and some weathering experiments. Kudos to the first person to identify the source of the head.