It's unlimited but not headache free.
Note that the Amazon Prime Photos app will back up (as it's a valid image file) but can't display a preview of the RAW files Samsung ExpertRaw uses as it doesn't embed a preview. It's fine with the standard RAW from Samsung Camera App Pro mode though. It's quite annoying.
It can also be fussy with naming suffixes, so you have to rename without () If you have made multiple versions of an image.
It will let you zoom on a JPG, HEIC, DNG (Pro or 3rd Party App) however if you have ever save any of your edited high resolution images out to PNG it can be a bit weird displaying these.
Full screen previews can be a bit slower to load than Google Photos as well but it's easy enough to download the original.
Edit: Forgot to mention it also doesn't support Samsung motion images. I don't use live images so I hadn't thought of them but it's a shame as it does with Apple's.