There are many cases of lost photos in iCloud, and Apple usually cannot recover these photos completely. If you use iCloud, it is recommended that you keep a copy locally.
I understand completely. We recognize that not every user is a tech enthusiast, so providing excellent usability for non-technical users is really important to us.
Yes, it can handle very large photo and video libraries! Users have reported successfully backing up over 20k photos and videos without issues. I don't have 150k photos and videos for testing.
Currently, direct backup of photos and videos stored only in iCloud is not supported. I will consider adding iCloud integration in a future update.
The original album and photo information will be preserved. Photos and videos will be organized into backup folders by Device Model/Year/Month.