Acem2 was my first experience with a private server so I'm not really an expert on the subject of low rate vs high rate but surely it comes down to content more than anything.
On Acem2 it took me less than 2 months to reach level 50, and at level 50 I had every skill ingame at max level and from what I saw there was 1 ring and 1 necklace at level 52 which wasn't enough incentive to continue leveling. I'm certainly not bashing Acem2, I enjoyed it immensely but after 2 months of gameplay I was coming to the end of what I perceived as content.
In Mir 3 you gained a lot each time you leveled (hermit stats, etc), but in Mir 2 the diference between a level 50 warrior and a level 55 warrior is so minimal there really is no incentive to leveling other than epeen and the ability to shoulder dash more players.
IMO it comes down to reward vs effort. You either need to offer more of a reward to leveling at 50+ or you go the high rate route to make it less of a ball ache to level. And ofc with high rate you'd find it harder to provide newer content to match the levels.
