I do wish it supported encrypted storage. For example, I wrote/maintain a Vault plugin to do basically the same work as the backend side of this project[0]. I wonder if you would be interested in supporting Vault as a backend in addition to PostgreSQL down the line? Feel free to reach out if so.
To answer your question:
Like some others here, I haven't found the actual integration points to be terribly difficult with most OAuth 2 servers. Once you have a token, you can call their APIs. No problem. I wrote the Vault plugin I referenced above to basically just do automatic refreshes without ever exposing client secrets/refresh tokens to our services, and it works fine.
Rather our customers would get into situations where they inadvertently revoked access, the user that authorized the integration initially left the company and it was automatically disabled, etc. and there was no notification that it happened. Basically all of the lifecycle management side that couldn't be automated down to "refresh my token when it's about to expire" sucked. So anything you're looking to support there would be a huge value-add IMO.
Another one is that each provider has their own scope definitions/mapping to their APIs. Some scopes subsume others (e.g. GitHub has all repos, public repos, org admin, org read-only, etc.). Some get deprecated and need to be replaced with others on the next auth attempt. We could never keep them up to date because they were usually just part of docs, not enumerated through some API somewhere. If you had a way to provide the user with a way to see and select those scopes in advance, that would be huge. Think if my app or a user could answer the question "I want to call this API endpoint, what scopes do I need?" by just asking your service to figure it out.
[0]: https://github.com/puppetlabs/vault-plugin-secrets-oauthapp