If in SPM you put in some bad project config, you'll get an error.

XcodeGen seems to accept valid YML.

For example I added the line:

thisshoulderror: nope

and Xcode gen was just happy to ignore it!

⚙️  Generating plists...
⚙️  Generating project...
⚙️  Writing project...

This approach is fundamentally flawed and should never be adopted in software development.