Many people suggest that you can auto-backup with whatever other tool works, and indeed there are a few choices.
But that misses the point, which is that in Google Photos you can manage the same collection from the phone or from the web. Adding a pic to an album should feel the same, no matter whether the pic is already synced or not yet synced.
This does make sense as a more specific UX/workflow complaint, google photos and similar setups are going to do much better with that. You can use the PWA on mobile after the sync runs, but it's not going to be the same as native apps on mobile and server that manage syncing behind the scenes. So definitely a downside if most of your photo management happens on the phone.
But that misses the point, which is that in Google Photos you can manage the same collection from the phone or from the web. Adding a pic to an album should feel the same, no matter whether the pic is already synced or not yet synced.