What about data protection? Does data go on the internet? Which data? Are you dependent on external servers or can you operate the server yourself? Do I have to create an account somewhere and disclose data? Which data? What about in 10 or 20 years, when the company and its server no longer exist, will we only have electronic waste?

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Accepted Answer

Things are bad with this one.
All data goes over the internet. 
In case of connectivity with the bridge, i.e. with WLan and not with Bluetooth, JSON dumps containing among others your address go via HTTPS to the Nuki servers. They will then open your door if you have sent the correct "secret". If you open the lock with Bluetooth only, the data will be sent via your mobile phone, i.e. via the app.
You have to create an account and are dependent on their servers, so if the company goes bankrupt you are out of luck... see Nello, a company that also specialises in making door openers smart and went bankrupt last year and was able to sell itself at the last moment to an Italian smarthome company (Sclak), of course with all customer data. 

In short, you pay for convenience not only with money but also with your privacy and security.

Answer

I was able to find out the following, operation in LAN-only mode should always have been possible, all you have to do is set the bridge to "developer mode".
You can then operate the API manually, or use any smarthome implementation such as https://www.openhab.org/addons/bindings/nuki/.
Additionally, you can save the $55 for a "bridge" as long as you have any old (bluetooth-capable) smartphone lying around, you can just run the "Nuki Software Bridge" on it.
I haven't tried this myself, because I wanted to know before I buy such a device, but the whole thing sounds credible enough that I would dare to try it.

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