Somewhat new to Homebrew. As I understand it, Homebrew Cask looks for apps in its Github repository. I don't know how they get there, but I assume that the app or the link which lives at Github is not supplied by the developers of the associated app. So, then, is a download from the Cask repository an exercise of trust in the Cask maintainers?
Yes, and also trust that having the sources in the open and others using it, you don't need to be the first person to catch a bad act. The traceability of who checks in what hopefully is a deterrent to casual maliciousness.
You of course are more secure to not automate installs of unsigned software from the internet.