After playing in a 5e game for a while now I have come to detest Milestone Awards for xp or leveling. In the past I have played in D&D games where xp was awarded through 'milestones', yet it was always xp that was awarded and not direct levels. I have also played in skill based games where xp was awarded based on story milestones. In both cases I tolerated the practice, yet now with a new perspective I see the perniciousness of milestones in conjunction with D&D.
The root of the problem applies to all RPGs in general; milestone awards are given out when the DM feels like it. The advancement of a PC is entirely dependent on the whims of the DM, granted that could be said for every other method of XP awards, yet with milestones a player has no real choice in the matter. Using milestones to level up a D&D robs a player of their choice to do things that will advance their character. What constitutes a 'milestone' is always an uncertainty to the player, and so it can seem as though anything they do has no consequence.
Level based games, like D&D, exacerbate the problem. In D&D it can take a significant number of sessions to level up when using standard XP awards. In skill based games, some XP is usually earned every session, and so even if the awarding of the xp is arbitrary and based on milestones, the player can still feel invested in their character. When a DM doesn't ever give out XP and just tells the players when the PCs have gained a level after 4 or 5 session or some arbitrary story point is passed, the significance of the increase in level is lost. That sense of reward and gradual increase that comes with every single session is gone, and the new level is just some new numbers on the character sheet. I don't feel like I earned that new level. And no matter how much work we put into the game as players, we have no idea how close we are to leveling up. There is no longer the incentive to do a little bit more so we can get that last 100xp. And in addition levels don't matter, not just because everything is scaled based on challenge ratings, but because being level 10 doesn't feel any different than level 1; my investment in my character hasn't increased over 10 levels. I don't feel like my character is more powerful. There are different numbers on the page, but we haven't changed our behavior in the game world. We don't act more powerful or experienced and the NPCs don't treat us any differently.
Level based games, like D&D, exacerbate the problem. In D&D it can take a significant number of sessions to level up when using standard XP awards. In skill based games, some XP is usually earned every session, and so even if the awarding of the xp is arbitrary and based on milestones, the player can still feel invested in their character. When a DM doesn't ever give out XP and just tells the players when the PCs have gained a level after 4 or 5 session or some arbitrary story point is passed, the significance of the increase in level is lost. That sense of reward and gradual increase that comes with every single session is gone, and the new level is just some new numbers on the character sheet. I don't feel like I earned that new level. And no matter how much work we put into the game as players, we have no idea how close we are to leveling up. There is no longer the incentive to do a little bit more so we can get that last 100xp. And in addition levels don't matter, not just because everything is scaled based on challenge ratings, but because being level 10 doesn't feel any different than level 1; my investment in my character hasn't increased over 10 levels. I don't feel like my character is more powerful. There are different numbers on the page, but we haven't changed our behavior in the game world. We don't act more powerful or experienced and the NPCs don't treat us any differently.