Mu's combos aren't hard, but playing her right is. She's probably one of the most difficult characters to learn in the regard. Like pochp said, it takes awhile to get a feel for when you can put steins out safely. Without her steins to defend her she becomes a bit of an easy target. You'll get stomped over and over again when you first start off playing her because she's a very thought heavy character, one you have to know inside and out to be effective with her. You really gotta know how far all her normals extend and know what distances are safe to poke from. You gotta know where your steins are at all times too. If you're not careful you can actually screw up your own combos if you're not paying attention to where your steins are.
Theres also a lot of making shit up on the fly and adjusting to the proration when you pick things up off of various laser hits and explosions. This is another thing that makes her hard, because you really have to get a feel for when something will work. It's real easy to blackbeat if you try to do too much stuff off of random hits, so knowing her move proration is also really important.
That being said, if the only real reason you're thinking on switching is just for the Lam matchup, don't bother with Mu. She's a character that takes a lot of time and dedication to play right. She has SO much stuff to learn, and still so much stuff we don't know (*cough* Hey C0R where dat stuff you promised? )
And this one may not really be truth, but my own personal opinion on her...to me, Mu isn't a sub character. She is a main, because you WILL spend more time figuring her out than any other member of the cast. After months of training, countless casuals and lots of time I still feel like I'm a Mu newbie. I know people do sub her but unless you spend a great deal of time with her she's never gonna be up to par imo.
My 2 cents. :3