For me, the Universal Clipboard feature of Continuity has always been a bit spotty, but with newer hardware, it has become somewhat more reliable.
The problem is getting it to work consistently. I’ve read the Apple Support document, Use Universal Clipboard to copy and paste between your Apple devices which unfortunately is hopelessly simplistic.
Yes, I have Bluetooth, Wi-Fi, and Handoff enabled, and all of my devices are correctly signed into the same iCloud account. Other Continuity features (Handoff, notably) appear to be working just fine.
However, what I consistently experience now, between my iPad Pro 11” and my iPhone XS, is this: When I try to copy on one device and paste it on another, what happens is that it won’t work at all the first time, and then if I repeat the paste, the device on which I’m pasting will display an alert “Pasting from … ”, I’ll have to wait about 5 seconds, and then the text will be correctly pasted.
Previously I had seen this message appear when pasting a rather large text (say a page), but now it occurs repeatedly even when I’m only pasting only a few words. This is, however, not the case between the iOS devices and my 2018 MacBook Pro running macOS Mojave 10.14.5.
I've posted about this earlier — with different (older) hardware:
In the past, the most reliable fix for the Universal Clipboard not working at all — as reported on various sites — was to sign all devices out of iCloud account, restart, and log in to iCloud again on all devices. Yet as noted above, Universal Clipboard is working to some extent for me, while signing out of iCloud and signing in again is not as easy as it seems -- many settings have to be re-established.
Are these sorts of problems par for the course with the Universal Clipboard? or is there a clear way to get it working quickly and reliably? All of my devices are running the latest versions of system software (macOS Mojave 10.14.5 and iOS 12.13.1).