It's hard to tell how much it says about difficulty of harnessing vs how much it says about difficulty of maintaining a clean and not bloated codebase when coding with AI.

Why not both? AI writes bloated spaghetti by default. The control plane needs to be human-written and rigid -> at least until the state machine is solid enough to dogfood itself. Then you can safely let the AI enhance the harness from within the sandbox.

Were human organizations (not individuals) any good at the latter anyway?