I am using it, and I find it provides good services overall. Much more flexible than traditional banks. I contacted the customer service once and it responded.
I have not used the API, but I looked at it at some point and it seemed capable of doing what you could do in the official app.
Well, you can't have it both ways. Banks that follow the standard "we are a respectable organization of respectable managers" image rarely allow people a look into their systems.
Disclaimer: I'm a bunq customer, both for business and for private purposes. Smoothest experience I have ever had with a financial organization. Stuff "just works" and is hassle free.
I have not used the API, but I looked at it at some point and it seemed capable of doing what you could do in the official app.