- There is a standardized API for interfacing with banks, HBCI, which almost every other bank supports. Except N26, who stated that they have no plans to do so.
- Their app (which is the only real way to interact with the account - there's a web interface but it's clear that the app is meant to be the main way) used to be really bad. They gradually improved, to the point that I wasn't able to get an error message by just opening it, but the fact that that was a pleasant surprise is not a good sign.
- They repeatedly made misleading statements about the acceptance of their credit card (meaning that I'd be stranded at an airport had I not had another card), their support was generally slow.
- They lured users in with great conditions, then made them progressively worse.
The instant credit card usage notifications work well though, and since it's free... but I don't consider them good enough to use them as my main bank - I expect anything that goes even slightly outside of everyday mainstream transactions to go horribly wrong.
If they manage to gradually fix their issues, they could be great, but as of now, I feel like they're just another startup that wants to "disrupt".