I don't find the Underdark boring.
That said, it's not what I envisioned when I thought of the Underdark. I always pictured true darkness, a real absence of light except in the rarest of places. And I imagined tunnels snaking up and down through stone, occasionally dumping into large cavernous chambers, and branching off in maze like directions, with random drops that suddenly appear, going down to deep rivers that lead to dangerous underground lakes.
Instead it's this giant open area where you can see all around from the proper height, and there seems to be an abundance of light and large alien mushrooms. It's certainly creative; I won't take that away from it. It's just not what I was expecting.
There's plenty to do, so again, I don't find it boring. But it's not the same as the surface at the same time. On the surface, each area feels like a full area. There's the crypt and the grove and the blighted village and the hag and the goblin fortress and and and... Each area is engaging, and there are smaller encounters in between those areas.
I suppose that's the difference. Excepting the myconid village, a lot of the Underdark feels like those smaller encounters on the surface between locations. The hook horrors, the magical sword, the petrified drow, the kuo-toa... all of these encounters lack the fullness that's found in the grove or with the hag or in the goblin fortress or even in the Zhentarim base. So it ends up feeling a little more rushed, a little smaller in scale.
