Strictly speaking it is possible to use fonts other than R, B, I, BI using ".fn" but because the set of fonts is determined by the output device it's very limited.
Much to my surprise GNU groff's font format does actually contain kerning information and ligatures although it's quite primitive and none of the included fonts have characters much outside of Latin-1 (to the point that there's a specialized "EURO" font you have to use if you want a Euro symbol). I would have to go over the older documents I have to be sure but I don't think this was a feature of the original troff.