In a railroad game, even one using XP and levels, where the DM serves up the encounters, they have a lot of responsibility to make the fights tight and tense. Too easy? That was the DM’s fault. Too hard? That was the DM’s fault. Even to the point that some DMs in this style might be tempted to put their fingers on the scale a li’l bit and fudge some things.
A game that uses milestone levels, regardless of whether it’s railroaded or if it’s wide open, has the same problem. The DM needs to make sure the players get the tools they need to face the opposition they take on. In fact, the problem is bigger, because the DM has all the same responsibility and all the same issues, but fewer tools at their disposal since getting really really good at encounter design isn’t enough if the player characters haven’t been leveled enough.
A sandbox combined with a traditional XP curve, and it’s got to be a curve, solves this. It’s suddenly up to the players if they sent their characters to an area that was too easy (and not rewarding enough) or too difficult (and too deadly). This is one of the genius inventions of D&D, even though I’ve seen it best executed in well-designed JRPGs like Final Fantasy Legend. It braids a push-your-luck element into exploration.