My understanding is currently that bunq simply does not support "Online Banking" via HBCI/FinTS and also does not provide a German IBAN. This makes a decision for bunq for (bigger, more professional) German companies impossible or at least challenging. But missing HBCI/FinTS implementation is not the only problem – for all upcoming implementations, accounting software providers require that bunq supports PSD2 XS2A (e.g., through finAPI).
I'm not sure if just providing a cool custom API, and telling all the big players in the market can simply develop against it, is a promising business attitude. Nice to have a powerful custom API, but without proper standards support, it's not of much help for professional businesses.