I have a clipboard manager application called "Paste" (creative i know). Its an awesome app for a million reasons. But one thing I like is that it allows me to see and hear when my iphone copy worked.
So I have it enabled so there is a sound when something goes into the clipboard. Even on my mac, I have come to rely on that audio feedback. But it has the added benefit that when I am using my phone in front of my computer and I copy something on my phone, I immediate (and it is impressively fast... maybe a 200ms delay), I hear the chime that something was added to my clipboard on my mac. So it gives you that good feedback that a copy "worked".
You can also shift+cmd+V to see the clipboard history, which is another complimentary tool with universal clipboard because if a paste isn't working as expected you can see if the universal copy never "took" (as you mentioned it is semi-unreliable), or if it just got overridden. You can then use the navigator to paste the older item.
I've noticed that more and more apps on both macOS and iOS sniff the clipboard contents and randomly clobber it. I usually notice it in apps like Sourcetree, where I'll click something or do a certain action and suddenly I can't paste anymore. I even get a feel for it, like my mind detects the pattern that empties the clipboard so I sense when I can no longer paste, but I can't figure out concrete repeatable steps to make it happen. On iOS it's more random, and I feel like it's probably Facebook doing it, or maybe websites in Safari. I just assume that everything is spying on my clipboard contents now, hoping to log secrets/passwords and PII to sell to scammers.
I have to say, this is one of the more disappointing developments from Apple, that they certainly must know by now about these clipboard shenanigans, but have done nothing to stop them. They need to implement permissions that deny all apps the ability to get/set the clipboard by default, and have an option to ask the user whether so-and-so app can access the clipboard (outside of normal copy/paste), every time with the option to allow always. And all clipboard access attempts should probably get logged somewhere.
I would think requiring opt-in for clipboard functionality would be the more radical option that would leave most users (myself included, I would imagine) scratching their heads when they can’t copy/paste as a matter of course. Maybe you meant something more specifically related to 3rd-party sniffing/modifying clipboard contents, but I haven’t really encountered that outside of apps such as CopyQ and Paste, and they are pretty explicit and intentional about their functions.
I have found a lot of utility with cross-device copy/paste. I know it requires the somewhat mysterious phantom Bluetooth/Wi-Fi connectivity that AirPlay/Airdrop use, so if I have disabled Bluetooth on my device, for example, it will no longer work. I could see where it might not be fully reliable enough to count on, I have experienced inexplicable failures, not often but enough to understand that it might not be some folks’ default preference. As part of the “handoff” function, it can be disabled in Settings at least.
So I have it enabled so there is a sound when something goes into the clipboard. Even on my mac, I have come to rely on that audio feedback. But it has the added benefit that when I am using my phone in front of my computer and I copy something on my phone, I immediate (and it is impressively fast... maybe a 200ms delay), I hear the chime that something was added to my clipboard on my mac. So it gives you that good feedback that a copy "worked".
You can also shift+cmd+V to see the clipboard history, which is another complimentary tool with universal clipboard because if a paste isn't working as expected you can see if the universal copy never "took" (as you mentioned it is semi-unreliable), or if it just got overridden. You can then use the navigator to paste the older item.