I also have the 2018 mini and stick with my assessment of 95% success. Not really any better or worse than my 2016 MacBook Pro.
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I wish I could figure out what’s wrong in my case.
I always get an error message if it’s connected to a different device saying the WiFi signal is too weak. If I connect both to the same device it then works straight away. All the network devices at my work are extensions of the same network.
Maybe a Watch issue, or interference affecting BT/Wi-Fi. My watch is series 4
Yeah. Mine is Series 3.
It works perfectly for me. If I’ve gotten up in the morning, put on my watch, and forgot to unlock the watch, it won’t unlock my iMac, but that’s how it is supposed to work. The watch unlocking my iMac has been a big time saver for me.
Ironically I’m pretty sure mine unlocked the Mac while lying on the desk yesterday.
Maybe try toggling the Watch’s Airplane Mode off, then on when it malfunctions. That disables both radios (Wi-Fi & BT) and hopefully that may give you a clue as to where the issue is.
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If the Watch is not unlocked, your Mac will state that it must be unlocked to do the unlock
My series 4 works perfectly on my 2018 MBP.
Might be worth a call to Apple Tech Support to see if they can help you. This sounds lke some kind of settings issue.
You are aware that it is only designed to unlock the Mac if it has been sleeping or is otherwise locked, right? It is not designed to log you in after a startup or other system restart. In that case you must enter your user password first to access your Mac.
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No issues here working 99.5% of the time
This is really odd behaviour and had me second guessing my own experience. I just checked though, and my Watch doesn’t even connect to the WiFi at work (or at least its MAC hasn’t been on the WiFi network for the past 2 weeks).
Pure Speculation Mode ™: The Watch and Mac use frame timing to determine the distance between the devices. When the Watch and Mac are both on the same network then they use that network path for this determination and if they are connected to different access points then there is enough latency introduced to make the Watch “too far away” for unlocking to work. If they are not on the same network then they use an ad-hoc connection to determine distance. (This seems unlikely to me, as I would expect them to always use an ad-hoc connection for this purpose, but it’s a reasonable hypothesis that explains our differing experiences on at-work networks).
I’d say 99% of the time on my Mac Pro and 95% on my 2017 MBP. Love the feature.
80% of time here (mid 2013 MacBookAir and Series 1 - or 0.5? the empowered series 0 that came out with the 2nd gen in 2016 - watch).
looks like it’s a mixed bag of a feature…
Works at least 95% of the time for me.
I’m using iMac 2018 with Mojave and iWatch series 4. The only time it has not worked is when I shut down the iMac. It always requires pw on boot. Then, as long as it is only sleeping, it has never failed.
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It’s not supposed to work between reboots, so yours is working as intended. Similarly to TouchID or FaceID, the watch can be used to unlock but not login to a device. (I’m only mentioning that because I’m not sure from your post if you’re aware of it 
)
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Thanks, I should have been more clear, just trying to be helpful – you get a message explaining the behavior each time it requires the pw in Mojave. Good time saver! 
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Works for me but I turned it off because sometimes I don’t want it to unlock just because I’m close.
Works for me most of the time (~9/10). I’m running a series 2 Apple Watch and a 2019 Mac mini.