Having the Homepods set as the ATV default output speaker as of now, only have one (big), advantage over selecting them via the ATV control center: since it routes ALL of ATV audio through the Homepods, it can bypass some Airplay audio limitations, like using the Homepods with Apple TV video games or IPTV/live tv apps (like Sling), which currently doesn´t support regular Airplay audio because of the latency.
Given that choosing the Homepods as the default ATV output creates an ad-hoc, improved airplay connection, this latency can be eliminated, which makes this the optimal way to use the homepods with your ATV.
However, as you say, currently the implementation of this feature (default audio output of an ATV 4K), is very buggy, and selecting the Homepods via the control center airplay menu is adecuate for most uses.