I’ve been a happy user of Google Photos for the last year or so. Prior to that, I was persevering with Apple Photos (formerly iPhoto) but the macOS app had become very buggy and even basic tasks like sharing albums with other Apple users was very hit-and-miss (good luck trying to share with a non-Apple user). The Google Photos web and iOS apps are great, and most people have a Google account so sharing is really convenient.
I trust Google to do a good job of looking after my photos but it seems sensible to have an offline backup too. One way to do that would be to enable the Google Photos folder in Google Drive. You could then install Backup and Sync to sync the photos to your computer and then backup as normal (e.g. using Time Machine). A small snag is that I don’t have enough disk space on my laptop to download all of my photos and Backup and Sync is also a bit crashy.
Fortunately, Google announced a new Google Photos API back in September. The API supports everything I need to build a basic backup program as well as uploading photos and creating albums which might be useful for future projects. Unlike Backup and Sync, I can run this program on a Raspberry Pi and backup everything to an external HDD.