That's weird because for me, read message are default and unread strong. You mean the difference in weight is not enough?
Also, we can inherit both strong and salient (maybe in that order or reverse).

For me I see no difference at all between read and unread. At least on my monitor and set up (macos 10.15 without font smoothing) I can't tell the difference. I'm using size 12 for the fonts though, so maybe that's the issue? Deriving from both would perhaps be an even better idea.

Oh I see. On BigSur, there's a bug in font selection and you cannot set the light/regular weights. That might be related. What I did to temporarily fix the problem was to derive 2 different fonts from Roboto Mono to have the right weight. I can upload them to the repository if this is also the bug for you. Best would be to starts emacs -q and select Roboto Light and Roboto Medium and check if you see any difference.

I'm not affected by the bug, as I'm still in Catalina. but somehow I couldn't see any difference. Also, do you know if different weights also have different heights? I can see fonts jumping sometimes if they change weight - see gif below:

Does this happen only at my end?

There's something weird with your fonts because the weights should not affect the baseline. Can you check in textedit (or Pages for example) if the bug is the same?

No - it seems the issue happens because I wasn't loading nano-layout. That first sexp setting the default frame parameters seems to be required for properly setting fonts. Apparently otherwise the default font size leads to different weights having different heights.

I'll soon add a nano-typography.el file to separate the font definition that will hopefully solve your problem.