I never thought I'd stir up such a lively conversation when I started this thread ...
But just to clarify my point, there is definitely a difference between opacity and fill, especially with the blend mode "vivid light" which I often use for glowing effects.
Here are two examples with four colors (yellow, magenta, cyan and black) on a colorful background and one with a neutral grey layer with a soft white line.
Whereas I get vivid colors and an oversaturated look with fill, using opacity makes it somehow dull looking. NOT the same.
Photoshop:
Here is the same file opened in Affinity Photo. Due to the missing fill functionality both sides look the same:
Imho, there's no way to replicate that effect in AP.