I'm not aware of any control over what gets backed up.

I'm assuming that all of my local work would be lost. But I'd still have the sixteen million duplicate photos.

iCloud's priorities are not a great fit for preserving my source code repos. Fortunately those aren't very big.

But when my son's Mac dies, his huge Logic Pro projects are toast. And he is a few thousand miles away now.

Kind of, but without a sub you only get a trivial amount of storage.

There is no iCloud Backup for Macs, only iOS devices.

There is iCloud Drive sync, but that only syncs your Documents and Desktop folders, but any other files are not synced.