I own a nexus 5 and a nexus 7. I believe the NFC unlock app writes something to the private inlay that links it to the NFC chips of that particular device. Thus from this standpoint I suppose I can't use the same ring with another device, because then the unlock app on the other device will write its own id on the inlay (in other words the ring would be useless with the 1st device). Am I right ?

I had a very similar problem yesterday on a Nexus 5 with Android 4.4.2. It took an arbitrary amount of time (in minutes) for the ring unlock to be activated. I thought it didn't work initially but after a while it activated itself.

Now, because I have a professional account on my phone (or at least I believe it the cause), the ring unlock hasn't replaced the PIN. Thus when it activated itself I had a kind of double security, first the PIN code then the ring swipe. I can't remove the PIN protection because my professional account is set as admin and it has deactivated some security features (i.e. I can't remove the PIN check). I tried to unset the account as admin, but then it would basically swipe all its data so not an option.

This configuration is probably not common, but I wonder if it's not a trouble maker. Basically what happened after a while, once I entered my PIN, the ring unlocker never recognized my ring (even though I was hearing the noise indicating a successful swipe). Since I have a 2 factors authentication enabled on my Google account I was locked out of my phone. I must say I didn't think about temporarily deactivating the Google 2 factors authentication. Also I didn't try to generate an application password which is one of the features of Google 2 factors authentication (i.e. for apps that do not support it you can generate a password that allows to bypass the 2 factors authentication and simply use a standard login / password). Reading on github I understand that setting the app password allows another recovery mode but didn't know at the time. What I did is trying to unlock when calling myself. I saw that in the app there where specific parameters when a call occurs. It didn't work either. Then suddenly after one more try the phone unlocked and I had 22 open pages on Chrome corresponding to all my attempts to unlock the phone (it opened the link stored on my ring). I have no idea why the phone finally unlocked or why it didn't before, but I was unable to use my professional phone for one day. Overall it makes me wonder about the real security of the whole system.

Pretty scary. I was close to try a hard reset to get rid of the ring unlocker.

Also if I may when an app asks me to enter my Google credentials, I'd rather be in a browser page. It's not the kind of credentials I like to share with an app especially on an odd red screen that doesn't work in this case because of 2 factors authentication.

Now that I've red some more I'll perhaps give it another try (I quickly removed the app after that episode), but I don't believe most people want to get problems with an app and be locked out of their phone.

I had the exact same issue on a Nexus 5 when trying to add a twitter user link to the ring. I simply entered @twitter_id and it created an invalid URL starting with en like "enhttp://twitter.com/...". Manually entering the URL with the link action in the control app instead of using the twitter action shortcut worked.