I always go by the old school method of writing on paper but to answer the question QR code should convert the format in completely offline so if someone using an air-gapped device then it's same level of secured as writing on paper, but if the device can expose to internet while reading or even after we done we can't say for sure that our seed is completely safe with us.
With particular emphasis on safe,
Just how safe can this method to wallet key security be?
The idea about keys to a wallet of this nature is how it could be completely secure and then, having it in a QR code formate might be a very simple means but, its just a scan away from being hacked.
Meanwhile, most the mobile devices these days don't need an icon to be clicked for searching out the program behind a code. Just a flash of the device camera and the code is linked.
Now, does this automatically suggest you a wallet for download for you or itould just error out if you aren't scanning from a specific wallet.