With a Time Machine backup, your risk is pretty much nil. If you have a large external drive, you should have months of backups, not weeks. My 3TB Time Machine drive has a years worth of backups for me. You could also use a dedicated external drive for just your library.
The way OSX Photos is built, your original photos are stored away out of sight and free from any manipulation really.
Anything you ask Photos to do in terms of edits are done with files that act on a photo at time of export and for previews. The original file is not altered.
When you use the library, those original photo files are synced to the iCloud, making your photos technically safer. If you choose to optimize the local file own this Mac, then you would be back to having only a single original copy of each photo. In this scenario, that unaltered original is on the Cloud and the local file has been altered to take up less space.
Most issue with firing up a large library for the first time, is the length of time it takes. Most users are somewhat impatient and expect this to happen in hours vs. the days it often takes for large libraries. This can also tie up a router and bog down your home LAN.
I would say your risks are in the setting up of iCloud Photo Library, not in it's use. Be aware of how it works as well. Meaning removal of a photo on your Mac removes it from all subscribed devices and vice-versa.
