The Emacs harfbuzz branch built fine for me on Fedora 29 (no surprise). 
I saw no problems, not that I would catch many (the Arabic I saw 
displayed OK to me, but I don't read Arabic so am not the best person to 
judge).

One minor tweak proposed in the attached patch, for simplicity/clarity 
in the mainstream-Harbuzz code.

I see that Harbuzz is disabled by default by 'configure', and that 
there's an environment variable to disable it at runtime. I propose 
removing the environment variable, and enabling Harfbuzz by default at 
'configure' time (if the Harbuzz libraries are available), as I don't 
see much point in nudging users away from Harfbuzz.

I see there are some FIXMEs and suchlike comments. Will someone be 
working on them?