I never ran completely through this but have used it recently. I ended up taking a different tact to it.
I photocopied the maps at about 10% to have the maps on 8.5 x 11 sheets of paper. Then, I went through and made an "ecology" of the various levels. I put the known monster groups in those areas, gave them enough rooms to have private rooms, chief's room, etc. Then, I explored the relationship that each area had with its neighbor. For example, the top left of level 1 is orcs. The top right of level one is goblins. I created some tribes to have a little intra group conflict (or said that a few tribes were gated in) and then figured out what the orcs and goblins do together, where they have conflict and what they are doing in general, besides trying to survive.
It was a lot of work, but now that it is done, I can leave it as is until a group fully explores an area. Then, I change around a few groups, remove a group or two and add a group or two and the next time they go in, the players know of some groups but the characters don't.
I liked that the map was open with no description, myself. I liked that the rooms looked like something, a temple or barracks or arena, as that helped define who was there and what they did. I had my own fight club near Skullport, with many of the factions of UM being present in some way or another, treating this as neutral ground.
A bit back on topic, then. Did I "run" UM? A long time ago, I did run it as a dungeon, with the characters coming in from the Yawning Portal, but it got old fast for me and the group. Plus, they didn't want to leave the site of the "entrance" and be trapped, which was probably wise! Recently, I ran it but the players had a direct way to get to Skullport and go venture from there. They did go to the slaver's area north of Skullport and a few other areas but most of it was Skullport. They were close to the drow area of the Promenade and had a few random encounters but otherwise didn't explore much. The focus was on Skullport.
I think the best use for Faerun's largest graveyard is more as a backdrop to a quest. The group has to retrieve something or someone from UM and they have to negotiate with the "locals" to get there. I don't see how any group of less than 14th level could otherwise survive for long in UM!
I hope this helped!
Have a good one! Take care!
edg