Asking about sound quality in these forums (fora?) presents a bit of a minefield (probably as with anywhere else). First of all, there seems to be a distinction frequently made between what audio nerds would call “sound quality” and something else that encompasses more catastrophic problems. The audio nerd’s “sound quality” would include notions like dynamic agility, soundstage size/dimension, tonality, detail, imaging, etc. The other way the term “sound quality” gets used here, especially by the Roon staff, encompasses more obvious problems/issues, such as drop-outs, stuttering, etc. My guess is that the Rule 5 statement about wi-fi is probably aimed at that second use of the phrase “sound quality.” You might experience drop-outs.
It seems that your questions are more about the first sense of the term. I don’t think Roon staff will take on your questions, if that is your meaning. They don’t seem eager to have those discussions, and who can blame them? Isolating one thing, such as wi-fi or RAAT or USB or whatever, is always an all-else-being-equal endeavor. Problem is, nothing is ever equal when it comes to sound quality (in the audio nerd sense).
If I was to answer your question, I would first say that the quality of the associated equipment might be dispositive of your questions. A pair of $500 speakers, and the other equipment normally associated with such speakers, is unlikely to reveal sound quality differences between wi-fi and ethernet. How much do you have to spend on a stereo to hear that difference? I have no clue. I could not hear differences in speaker cables on a pair of $5500 speakers (purchased in 2000) until I changed to a pair of $8k speakers in 2013 (not having changed anything else in the system). Really, my first answer should be to ask about how your speakers and your listening position are placed in your listening room (unless you’re rocking headphones).
I think the best way to scratch your itch (unless your itch is solely to understand why there would be a difference) is to try it both ways in your system. If you don’t hear a difference, be thankful and enjoy, or be poorer buying equipment that can reveal the difference. If you hear a difference, you should still ask yourself which one you like best. If you hear a difference, and yet further differences going from one wireless router to another or from one ethernet switch to another, then I will congratulate you on having a sufficiently resolving system, and I will wish you luck avoiding the madness that chasing the sound quality dragon can bring down on you.
It is so much easier to have this discussion if the only thing that “sound quality” means is whether or not the music drops out or stutters or there is some other basic failure. Yeah, if the hardware is working within specs, wi-fi sound quality is “good.”
I guess there is another issue lurking here: if you can’t hear a difference, is there hardware out there that can reveal it? In just about any context, usually the answer is yes, unfortunately. That can be a source of frustration or an opportunity for Buddhist wisdom. Go to an audio show and listen to the Sygergistic Research demonstration of their tweaks. Their sound is made blatantly obvious. The stereo that makes it obvious (in a noisy hotel room) probably costs $500k or more. At least with the wi-fi/ethernet question, it is easy to determine a difference in your system without having to spend hundreds/thousands of dollars. Bulk ethernet cable, although also blamed for sound quality differences, is not crazy expensive.
I hope this was somewhat helpful to you.
