My eero mesh network broadcasts both 2.4 and 5GHz simultaneously for the same SSID, and I have no issues with my X1C connecting. Previously I was using a Netgear Orbi mesh and also no issues. I do not think there is a systemic issue out there for this, but there has been some compatibility issues identified in the community with very specific vendor firmware like Ubiquity.
Having said that, sure - the option of an ethernet port would be a welcome inexpensive option for them to provide on future models or hardware revisions. I do not see an official retrofit becoming available for us though. I work with other maker devices like Glowforge and Ditto that also do not offer an ethernet port - wifi only seems to be a trend in this space.