The 4th Commandment of Payment by Results: Thou shall not covet milestones
The introduction of milestones exacerbates the problems of complexity highlighted above, requiring even more time and resources from commissioners and providers and even more onerous data collection systems. But even worse is the fact that as soon as you have indicators, people start focusing on them, instead of the intended outcomes.
Which takes us straight back to the culture of micro-management by targets which PbR is trying hard to leave behind.