Any of the 'optional' parts of the HDMI standard like ARC, CEC etc. will never be dependable or consistent. They don't always get interoperability 100% right for stuff everyone depends on like EDID and HDCP, so there will always be a heavy element of luck getting something like ARC which is used by a couple percent of people working.
Even if you get it working today, a firmware update on either end may break it tomorrow. At least with the features everyone uses if something is broken it will be noticed by many people and (hopefully) quickly addressed. The more 'niche' the HDMI optional feature you depend on, the less likely that is.
Digital audio is just as good as ARC in pretty much every way except hassle, where it is 1000% better than ARC.
