The thing about Underdone is just that it needs to be worked on and customized.
Sleeper's server had mostly unmodified content (maybe a few more quests, or something), that's why it went boring after you got the rifle that just pwnt everything.
Without people WILLING to keep Underdone alive and coding stuff for it all the time (not just 5 low-level quests and then tell players to gtfo in the world), it's dead after very few days.
And it's laggy. VERY laggy.