What are the challenges involved in creating a financial data aggregator of aggregators similar to Segment?

There are a couple of startups trying to do something similar and all of them have different takes on the space. Fintech is quite different because you're talking about consumer permissioned data and not just a data infrastructure play. 

For example, an end user would never know Segment existed because it works behind the scenes. However, they would have to go through a workflow to connect their bank account with Plaid. 

Plus, at the beginning, all of the APIs are closed source, so most of the value comes from reverse engineering those closed source, closed APIs.