I was delighted when I noticed the new Accessibility working mode in a recent beta! Bravo to the team for making accessibility more accessible for all of us!
From the observed behavior on my test site, I got the impression that it was only considering the contrast of the selected element. Images won’t pass WCAG without an alt label, so if it is not contrast it could be that. This is the cheatsheet I use for WCAG compliance: https://www.w3.org/WAI/WCAG21/quickref/
I didn’t get the impression from the original post or the testing I did that that the Accessibility Working Mode “check” was going beyond alt tags and contrast ratios (for example, I didn’t notice any errors related to keyboard traps on my pages).
I have used a few accessibility checking services over the years, but I find the best first few runthroughs to be on your own device… preferably an iOS device using VoiceOver and no touching the screen.
If you have a situation where regulatory pressures demand more of a professional solution, I have had a good experience with SayYeah.com -however, their pricing has raised significantly since I last had a need for their services, so there may be better options.