Full disclosure, I work for Capital One now. So... C1 has stated publicly it's interested in this space and some of its prominent employees are in prominent public technical groups trying to make it happen. The liability and fraud stories are really unimaginably complicated.
For people looking to make a product today, I've decided to add a quick primer:
Finicity, FinancialServices, and Plaid are all financial aggregators that you can use in combination with novel (read: quasi-legal) mechanisms you can use to provide financial services within the confines of the beleaguered law.
In general a financial product's engineering cycle looks like this:
1. Find a data aggregator you can partner with and give most of your profits to. 2. Work very hard on experience driven products. 3. Find a good lawyer who can help you produce a novel interpretation of the law that works with a specific financial product. 4. Still have limited success because fintech products have linear adoption curves (even Mint).
Examples in this space of novel mechanisms that use existing payment services, data aggregation products and very clever lawyers: Acorns, Digit, Tally.
I was one of the founders and the founding engineer of Level, and worked on Simple. My general observation is that most of the challenge is around data quality and aggregation on the technical side of things.
A lot of the challenge of making these apps work is balancing your fraud risk with your customer experience. Customers are apathetic and inclined to ignore the space, fraudsters are diabolically clever and very engaged.