I find undermountain to be practically necessary for games with a bunch of people; arguably Skullport, too. Undermountain just adds a few more handy places to place your workers, plus more intrigue and mission cards and buildings you can build. (They have a little beholder on them.) Plus a few more lords to work for, one of which (Blackstaff) rewards you extra points for fulfilling Undermountain missions and/or Undermountain buildings.
I haven't found the new Undermountain things to be broken.
Skullport, aside from adding more missions/intrigues/buildings to play with, adds the corruption (or skull) mechanic. I find it to be interesting, but because the skull mechanic deducts points at the end of the game I sometimes use the mechanic in reverse and stay away from gaining skulls while letting other players be distracted by the temptation to juice the game while adding skulls!
Thus, even if you prefer to ignore the skulls yourself, it can be a good idea to have Skullport just to tempt other players into losing!
(Plus not all of the new skullport cards actually give you corruption. In that sense they're just some extra opportunities to score and shift the game around.)
To wit: Skullport, aside from adding a few new locations and many new cards, adds the corruption mechanic by introducing a limited number of skulls into the game. If players take advantage of special opportunities to gain extra pieces or other extra rewards, they'll also pick up one or more skulls from the sideboard into their tavern. Each skull still in your tavern at the end of the game will count x-points against you; but the cost of having skulls can go up or down based on how many skulls are out among the players, or waiting on spaces to be picked up (which represents how 'corrupt' Waterdeep has become).
If only a few skulls are distributed, each skull counts only one or a few points against you. If most of the skulls are distributed, each skull will count 8 or 9 points against you! (And if you try to do something that would give you a skull, and none remain, then you permanently lose 10 points.) But you can do various things to put skulls out of your tavern: put them back on the skulltrack (reducing the amount of penalty skulls are worth to everyone at the end of the game); or give them to someone else (rarely); or put them as traps out on the gameboard forcing people to pick up skulls for visiting an area (uncommon); or sending skulls out of the game completely (rare) which in effect raises the minimum cost of having skulls at the end of the game.
I like the skull mechanic; it adds a nice level of extra calculation into the game's strategy, whether I'm using the opportunities that give skulls or not. It's admittedly kind of hard to describe.