You can't really compare Stingle to Google, Apple, or any other cloud photo services. Stingle's whole value proposition is encryption. Google and Apple don't offer encryption-at-rest, and Google in particular is using the photos you upload to learn about you and market to you. If you're looking for cheap cloud hosting and don't care about privacy, last I checked, Google offered unlimited photo storage (with restrictions on individual photo size that are perfectly fine for non-professional viewing and use).
No E2EE for photos saved in iCloud, but they're encrypted in transit (SSL/TLS) and at rest (they get encrypted on the servers and not your device so not using E2EE).
You have to scroll a bit further down to see “End-to-end encrypted data” on the page you linked :)
It depends on your paranoia about attack vectors. From photo EXIF data you can figure out what phones and cameras you use, where you took the pictures, and dates and times you've traveled. With AI or looking at the photos you can ID people and other personal details. I sometimes take photos of personal documents that can be OCRed.
Sure, techies tend to be more paranoid than the average person, but I do think people undervalue the long tail of data. Years ago I was talking to my wife about avoiding sensitive info in email with our tax preparer. I mentioned that we could send an email and a decade later he could get compromised and it would be trivial to scan old emails for specific data like w2s, social security numbers, or bank accounts.
