First of all, the scale shows the main numbers in increments of 20, which is already quite wrong - it should be in the increments of 10 for default zoom level. To get to increments of 10, you need an extreme level of zoom, more than 300%, which is ridiculous.
You can't really work at that zoom level.
Second bug, the hashes between main numbers are marked wrong.
On a normal ruler, between numbers 0 and 20, the longer hash marks would indicate 5, 10 and 15mm positions.
This doesn't happen in Affinity. Instead it highlights numbers 4, 8, 12 and 16 with longer hash marks.
This doesn't make any sense.
As a result, if you need to see where numbers like 5, 10, or 15mm are located, you can't. They aren't marked.
Here are many examples of how the correct mm ruler should look like
And here is how mm ruler looks in Affinity
Here is an example of Adobe Illustrator ruler at 200% zoom