I have made couple of comparisons of my websites. It works fine. It compares screenshots (pixel by pixel, source code and network request). When the core is updated or a theme, definitely the source code changes. But pixels should stay. The page marked as changed in the report and you can open the detail for this page and see screenshots with changed pixels marked.

You can add a link to sitemap to import all the urls. This is handy. I would not keep it without it.

The process is good. You create comparison, import the sitemap and run the first version scan. Than you can make any pauze you need. Then you can scan the second version. Versions can be the same urls, or a staging website vs production, whatever.

The scanning takes time, but it is on background.

What could be improved. It would be handy to keep going within the same comparison. I see the ability to rescan version 1 or version 2, so potentially you don't need to create a new comparison. I would not duplicate comparisons (it is possible). It would be better to have one place for all websites comparisons. Using previously saved version as a base for a new run. And ability to refresh the list of pages rechecking the sitemap.

But even as is, this is a timesaver. Recommend.