A couple years ago I started backing up following the 3-2-1 method. Until then I only had a local backup, with a small subset of images on dropbox. At that time, the largest barrier for me to use any of the online backup services was the slow upload speeds of DSL. I actually tried a couple, but I think the estimated upload time of my multi TB photo library was almost 5 years. A bit over a year ago I switched over to fiber provided by Sonic, great service btw, and was able to actually consider a cloud backup solution.
My current library is stored on a large USB 3 disk. I prefer keeping all my images on a single drive, instead of swapping drives searching for part of a catalog. Every couple of years my main drive will start getting close to full, and I will buy a larger drive and migrate my libraries. Smaller drives are then used as backup disks. I have Carbon Copy Cloner set to duplicate subfolders to smaller disks. I use Amazon Drive Photos as my cloud storage and upload after any bog shoot. Unlike many backup solutions, the Amazon is not automated or synced. To me, this is not an issue, as I view this backup as a snapshot.
My primary image library is not the only set of photos that I want to have backed up. I take more and more of my personal photos on my iPhone. Apple does make online pretty easy with iCloud. It is not free, the measly 5 GB they provide for free is woefully inadequate. But even if it were considerably more I expect I would need to purchase an upgrade. The local backup again is done when Carbon Copy Cloner creates its weekly backup of my hard drive.