I have a Synology NAS. I don't remember what I paid for the device it itself, but let's call it $300 on the cheap side + nearly $600 for the drives + $280/year to back it up (about $23/month).
That's a nearly $1,000 investment with a $23/month ongoing cost for backups, not to mention the added administrative (time) costs... and the eventual need for new drives and hardware (more money).... along with with migration costs (more time).
I'm also an iCloud Photos user. I pay $2.99/month for extra storage to backup all my photos/videos to the cloud and never think about it or manage my on-device storage.
The initial sunk cost of investing in the NAS could pay for 27 years of iCloud drive space (at the current price I pay), and the ongoing cost of my NAS backup is over 7x the price of what I pay for iCloud on an ongoing basis.
If it's a question of money at all, it makes 0 financial sense to buy a NAS to solve a simple photo management issue on a cell phone.
Then there is the technical requirements. Step 1 is getting someone to understand what a NAS is, which is where you will already lose 80% of users.
I have no delusions that I will keep this Synology solution for life. At some point I will need to migrate, which means solving the current problems it solves again. The big question is if I will be doing it proactively or reactively. If it's reactive, I will very likely end up forced into another Synology product due to vendor lock-in, unless I take active steps to mitigate that risk. Most users don't want to think or care about any of this; I don't even want to think about this stuff, but it's often on my mind.
The big risk I'm taking right now with iCloud Photos is that I have no idea how redundant the solution on Apple's side is. I hope they are good stewards of the data, but I really don't know. It would be a PR nightmare if they lost a bunch of people's photos. I have been thinking about looking into creating a copy of my photos/videos stored in iCloud on my NAS, but that is more time, and will likely require some kind of ongoing maintenance. It will likely also push me over a threshold for my NAS backup, and require that price to go up. I'm not excited about any of those prospects, so I've been trusting Apple for now (and for the last several years).
These aren't thoughts of "most users", and I envy them.