Missing unicode glyphs in PDF version

[rnhmjoj]

Pandoc 2 kindly started to warn when there are glyphs used in a document that are missing from the font, I didn't notice this before. There are indeed a few symbols (greek letters, double arrows, ...) that aren't being display at all in the PDF.

I have been battling with XeTeX to get it to switch font (using the ucharclasses package) on such symbols but I can't get it right apparently. Another option could be inputting those symbols using a math environment instead of unicode in the html sources but I'd rather not use MathJax.

If anybody with more TeX experience could suggest something it would be much appreciated.