@whitequark @wwahammy I think I'm one of them so I'll go over a few:
1. Resource usage externalities when this is done at scale, especially for large projects times large numbers of PR authors. This manifests as energy waste, hammering the servers/infrastructure of software you depend on and pull dynamically in standard "destroy the world and re-run everything from scratch" CI recipes, etc.
2. Dependency on subsidized compute resources from a capitalist platform with motivation to lock you in and enshittify.
3. Reducing or eliminating the mandate for your software to be independently buildable by people on their own systems without your CI infrastructure.