It took a while to sync my iMac Photos library to iCloud. Initially I was concerned that the folders on my iMac weren’t syncing to iCloud, but after the iMac had finished uploading all the photographs in the library, the folders were then created on iCloud. Though my smart albums weren’t synced.
While the iMac was uploading photographs, it wasn’t initially downloading the photographs I had been taking with my iPhone, which was also using iCloud Photos. However, as with the folders, once it had finished uploading, iCloud Photos downloaded the new photographs to the iMac.
Overall I am pleased with moving to iCloud Photos, it does what I want.