I've been wondering about this too for a while.
The one thing that struck out to me is that CrashPlan and Backblaze will allow you to pull deleted files, whereas I don't think iCloud Drive or iCloud Photo Library would have that ability. I currently pay $5 per month for CrashPlan, and the main thing I need backed up is photos (with important documents being in Dropbox). I could potentially replace CrashPlan with the $4 per month 200GB plan. However, I'm not sure how it would work for restores and how reliable it will be. I intend try it out at least (before I cancel CrashPlan).