From my experience with and against them, terrain and large friendly Formations can be your biggest challenges with the Mumuk in WotR. If you can get your Mumuk into combat with other Formations (both the Trample and stopping on top of the enemy Formation are great ways of getting early damage) then they can be pretty effective. But it's getting them there that can prove the challenge. The random movement range is a real PITA for me as it only takes a couple turns of short move to mess up a lot of your plans.
We had a game a month or so ago where we had a couple on the table and they proved more of a dud than anything else, but most of those issues were related to bad move rolls and limited movement options early on with terrain.
http://tac-me.net/index.php/component/content/article/86-lotr-battlereports/lotr-battlereports-wotr/80As noted by others it's pretty easy to take them out if your army is tailored for it. As my only significant Good army in WotR is centered around my Wood Elves I almost always have Legolas and Thranduil and can roll disgusting numbers of bow shots so I don't worry too much about them with that army. But when I've played them against some of our other locals they can be fun and worth their points as long as things go well. I did have a case where I blew my Courage test far earlier in the game than you should expect and my Mumuk charged into some terrain and was eliminated right off. I also find that my random stamped rolls almost always go right at one of my top two Formations and never into my opponent's Formations. I find the best option is to just blast it forward to break enemy front lines and make it easier for your real troops to leverage the weaknesses created, and then try to get it in among the enemy tight enough where almost any random direction has a chance of hurting them. And then just let it do what it can until it dies.
Our group has enough Mumuks among us (four or five) to try out the War Mumuk BattleHost and I hope to give that a go sometime in the Fall or early Winter. I think that will be really fun and dramatic, though I'm not sure what anyone would want to put against them.