Just to give my +1 to this topic: I have just encountered the problem specifically with Bahnschrift (which is a Windows standard font, by the way!) that was chosen for a project, and I can easily use that in multiple applications, including MS Office, Paint.Net, Inkscape, etc.
But I can not use it in Affinity Designer 2, because whatever I chose from the font menu, I always only get the standard cut, and can't select any of the other variants.
Now, I understand that this font has certain features that are not supported in Affinity – like that you can adjust parameters like weight very flexibly, ideally with a slider control. That's a great "nice to have", but in most cases, a more simplified approach would already be a big step forward to the situation that we have now.
I see that Designer seems to recognize that there are multiple default variations available from the fonts list - and it shows them, but without the proper name, and selecting any of these always sets only the default font variant. In other words: it is completely broken and not usable in the way it is now.
To me it seems as if Serif is not implementing a simple solution because they hope to deliver a perfect one at some point in the future - but this is not moving forward since many years, so maybe a simple, imperfect solution that could be rolled out in a reasonable time would probably be a better one.
As for me, I am now using Inkscape to insert this font into drawings, and then I convert all text to paths. And that is not actually a good workflow 😞
Edit: Sorry, I just noticed that this is the v1 forum, and there is already a new discussion on this topic in the v2 forum. Apparently I can't delete my post, so I just leave this here...