In my experience, they are "Kinda" balanced... Their strengths and weaknesses do on paper make for a balanced army.
The problem is that these armies are very specialliced. They only focus on one thing, and they do this thing extremely well.
In a normal 1 on 1 battle where you know your opponents race, these armies are fairly beatable, however in a tournament setting, the SOC lists can be tailored to be extremely effective against "All commers" lists.
While your army has to be atleast passable in all phases or aspects of the game to be succesfull, they can often ignore their weaknesses and focus only on their strengths.
They are not unbeatable, nor are they all evenly viable.. Some of them are however extremely good at what they are supposed to do
(the picture in my sig is from a SOC Slaanesh deamon army.. 2 units with move 5 and 8 with move 10 leads to a very fast army and fast game... Perfect for a slow player in a tournament