It's a marginal amount of work to link against a few big banks that have solid APIs, but once you get to the BECUs and the SESLOCs of the world, this gets much more difficult. Imagine doing this globally, and I think it's an even taller task.
The EU has been working on PSD2, which might make this less valuable, but there's still a long run way (IMO) where Plaid can quickly become the default choice for anyone who wants to link against banking APIs.
Beyond that, the network effect means that Plaid will likely eventually get to broker individual relationships with banks and be able to move into dictating more of the market in the long run, putting them in a position of strength.
I haven't worked with Open Banking, how is it? In the US, for bank transfers, NACHA files are fairly easy to work with, as long as you can access and verify bank account numbers (ideally get balance and identity information too). I think all the APIs support this, eg: https://developer.wellsfargo.com/apis/payments/account-valid...
It's hard to tell how quickly things will change and improve. Banks should have an incentive to work on this, since it will give them more control of their data and logins, potentially lowering liabilities and hack potential.