I think it’s fine as long as the user can opt-out and have the opt-out stick[0]. My problem with YouTube is that I can’t tell them “when I’m not on a metered connection, blast my face with as many pixels, frames and bits of color as possible” and actually have that happen.
[0] I say opt-out not opt-in because the audience for ABR is people who wouldn’t know where to find an opt-in button or wouldn’t know that it exists.
The "Enhancer for YouTube" browser extension has a setting to force assign the resolution to whatever you choose.
I was having issues that even if my player indicated it was playing 1440p, it clearly was at most 640p or something like that, complete potatovision. Re-clicking the 1440p option would fix it, and this extension basically does that for you.
[0] I say opt-out not opt-in because the audience for ABR is people who wouldn’t know where to find an opt-in button or wouldn’t know that it exists.