Doorkeeper is closer to a full-package with customizable features, including a basic frontend. I'm not too familiar with hydra, but it seems Doorkeeper is best when you want to get the full OAuth app & user interface running (and customize later), whereas Hydra is best when you want to get a quick OAuth API app and build your own frontend. Would you say this is accurate?

Yeah I think that is valid. Hydra can also be put on top of existing infrastructures. Not sure how well that is possible with Doorkeeper.

Doesn't the nature of an OAuth server imply that it can be added to existing infrastructures? Or is there an issue you foresee with non-Hydra libraries?

No, Hydra works with every existing solution :) You can read more on this topic in the guide: https://ory-am.gitbooks.io/hydra/content/oauth2.html

You didn't answer my question. I think you may have misread it.