I am honestly not sure. I use GG photo because that what i use first and i was too lazy to investigate better deal. Also, i know Google does really well at keeping my data available, which is what you want for backup.
Granted, an average person most likely will never mess up, but in google-land the surface area where to make that mistake is quite large.
I didn't check ToS of proton drive, but in my experience, every cloud storage has requirement about what kind of data is allowed to store(after all, they dont want to accidentally store illegal stuff).