Pvt. Winslow
Explorer
I do not think that he was saying that milestones are a pitfall, or that it is impossible to come up with rewards for players and characters that are not experience points or levels. The way that I read his post, I think he was saying that if you are playing with milestones, you lose out on a mechanical system to convince a bump on a log player from contributing to the game.
Well, I believe he was pretty clearly saying that the pitfall of milestone levelling is that the characters who don't contribute "by taking risks" benefit from the 'reward' of XP when they shouldn't.
If he meant as you believe, then hell, I agree. If you have a bump on the log player who needs mechanical incentive to even bother role playing or taking part during your session, then it sounds like they aren't a fit for the group. Instead, that's the kind of case where you simply talk to the player and gauge what they were looking for in the campaign, or you find story reasons to spur them to action.
Withholding XP from them because they didn't 'earn' it seems like a pretty antagonistic and frankly dick move for a DM to make. They should have talked to the player well before they felt the need to punish them for their behaviour in game.