True, but I'm not handling financial transactions.

It doesn't matter. With your definition of competence no one could be called that way ever.

A measure of competence is usually down to trust and demonstration of experience. We can take blockchain.info, a slightly similar service, and use them as an example. With no other knowledge I can see that their ready to go PHP developer examples have SQL injection vulnerabilities. That's not an issue with the service itself-that's closed source-but we now have a measure of the incompetence of the websites owner. There's no way of knowing what their system looks like internally, but that's enough to make me cautious relying on their data feeds as being 100% reliable.

It's quite possible to demonstrate robust systems.

But you have to have robust systems first.