I love the invoice scanning option but as mentioned I to am weary of uploading info like this to google. Or any other cloud service for that matter.
Most phones are powerful enough to analyse scans on the device itself. A nice example is Scanbot (http://scanbot.io). They also happen to make their SDK available for developers (https://scanbot.io/en/sdk.html).
It supports multiple platforms so iOS and Android users can enjoy more privacy if bunq would consider implementing this.