The current font dialogue is trying to filter out the non-fixed-width fonts, which is very nice, since the proportional fonts are far less suitable for use in Vim.
It would be worth considering to go even further with helping people see other relevant information about the fonts on their system right in the dialogue, so that the number of trial-and-error cycles to see how it works in Vim could be reduced.
The list of displayed sizes does not reflect the selected font family. The reason is that the "normal" style goes by many different names. The patch fontdialog_sizes fixes that for many fonts.
Some of the information that would be useful to see when selecting a font family: available styles, or supported scripts (writing systems)
Perhaps also whether the font is scalable (although one can guess from seeing the list of sizes, but that happens only after the font is selected). Maybe the list of families could even be separated into two parts, one having scalable fonts and the other having non-scalable ones.
There is also a problem of fonts with more generic or popular names. Qt reports them in the format "Family [Foundry]", and it seems that Vim will not accept this. I do not know in which form these could be passed to Vim (stripping the "[Foundry]" part is only going to allow to get one of possibly several fonts...).
Sometimes, fonts with glyphs for a certain language are stored as a "style" within a family (especially the generic "fixed" family). It is likely very difficult to select these now. Although the question is how many people actually use these fonts nowadays, and the automatic font substitution mechanism may be dealing with some of these cases.
With 4. and 5., if there is no way around it, maybe GUI could also display those fonts as not really available or problematic or different from the other fonts.