Well, I agree, the key and the hard part is making this easy to use and fool proof. But there is a big gap between a build in, simple to use rule system and developing and hosting your own app to achieve this.
I mean, alot of Bunq users would understand creating rules (maybe not everyone), but a far less amount of users would know how to develop this using the API. I'm not a developer, but work in IT, sure I could hack some thing together and it might even work, but then I really don't want to host my own stuff to do this.
And yes, you can get some one else to build it and host it for you, but then, I dont really trust a 3rd party moving my money around ;)