It makes it less atomic if you need simultaneous changes in multiple repositories.
> Had we been able to set this up as a separate repo where there were no global rules over how all compute jobs must be organized, and used our own choice of deployment (containers) with no concern over whatever other projects were using / doing, we could have solved it in a matter of a few days.
I think this was an organisational problem, but I accept the argument that monorepos will provide a seed around which such pathologies can crystallise. But I don't believe it's the only such seed and I don't think it's an inevitable outcome from monorepos.
