When G+ was still a great thing and Google Photos first came out I set up the photos backup to run against my raw files as a secondary backup to my main backup strategy. The sync app was horrible and it took months, but I backed up at least 200,000+ images. I had to let it run until I was approaching my former 300GB comcast data limit and then pause it until the next month.
When they poorly handled G+ and the updated sync app was even worse, I stopped using it. Now I only upload processed images manually for some family photos to share, but I still have the massive library of several years of raw images out there. Currently Google says I have 237,346 images on Google Photos.
You are right the facial recognition is scary good.
I love using Google Photos for searches. It is really good at finding things by location or what the image is a photo of. I used to use it often as a way to find a specific image that I knew about but didn't remember which year (I keep my LR libraries separated by year and file folders separated by year and event). I still use it some, but since my recent photos aren't there, I don't use it.
I'm not really much of a Google fan anymore given how poorly they handled G+ demise and all of the things the non-privacy friendly things that do now as you referenced. I also get tired of using a Google product that they have some people build 80% of the way and then abandon and eventually kill a few years later. I'm not going down that road again.
When Amazon announced unlimited I uploaded a ton of images there as well, but their uploader had no way to say don't upload files of this type, so when I pointed it at my pictures file tree I ended up including some non-photos and RAW files and hit my limit of non photos. It was too much work to separate the .jpg files form the non-.jpg files, so I gave up after it had synchronized 86,922 images (I just logged into to get the count). That was back in 2017 and I haven't checked to see if their uploader was more intelligent since then.
I hate how Apple handles photo synchronization and backup. I used to have an Android phone, but changed jobs and we only sell iPhone software, so I have an iPhone now. I have photos sync disabled on my iPhone and iPad because I hate how their sync works, so I don't have a library on Apple. I have Google Photos on my iPhone setup since it backs up the HEIF files in native raw format from your iPhone. I also hate that when I download an image to my iPhone, Apple strips all of the EXIF data and changes the date of the photo to the day it was downloaded. If Google Photos then backs it up, it ends up being on the wrong date because of this.