I just tested again this morning, here is my latest after I brought up 1 tab on Safari on each, and my main MacBook Pro has 4 tabs up:
MacBook Pro #1: shows other 3 devices correctly!
MacBook Pro #2: shows nothing on other devices!
iPhone: shows other 3 devices correctly!
iPad: shows other 3 devices correctly!
Then on MacBook Pro #2 I opened a new page on the same tab, it still shows up nowhere about 3 minutes later. Yet I then opened a new tab on MacBook Pro #1, and my iPhone and iPad show it correctly immediately. So at the moment, 3 devices re working fine, one MacBook Pro is not working at all.
One last test - I tried restarting the one MacBook Pro that is not working. I opened up Safari, and went to a new page. That laptop still shows nothing for anyone else, and the one page shown for it is from 20 minutes ago on the other 3 devices.
So it's getting closer to working, but seems to be at a different state of correctness per device.
EDIT: Now about 10 minutes later I thought to turn off iCloud sync on the offending MacBook Pro, close it, reopen it, and turn on iCloud syncing. Now, none of the 4 devices show anything from any of the other devices! Everything else (bookmarks, reading lists, history, passwords) looks fine. What a mess.