I am currently trying the new HarfBuzz renderer in TeXLive 2020 (using LuaHBTeX v. 1.12.0) with the STIX2 set of (serif/math) fonts.
Rendering seems alright with my main font (and I can confirm
mode=harf is used in the log file).
But rendering of math font is buggy: superscripts and fraction bars are either absent or misplaced.
Is this a known current limitation of the Harfbuzz renderer?
As per the comment just below, the HarfBuzz renderer is simply not supported for math because the math layout is always done by the engine itself and HarfBuzz doesn't seem to provide math parameters anyway (see https://github.com/latex3/luaotfload/issues/141#issuecomment-609932891).
Harfbuzz does not seem to have anything to do with math layout (see description) though it might be able to provide layout information for OpenType fonts (but it might be very limited): https://harfbuzz.github.io/harfbuzz-hb-ot-math.html