I like to think of Ceberus as barely existing at all, and not in a bad way. Like, I imagine, a lot of terrorist organisations a lot if it is loose collections of similarly motivated people taking credit for those actions as if they were some sort of cohesive organisation.
So some scientists start blowing up colonies on the outer rim and say they're Cerberus, some rich xenophobes with more money than sense give money to the organisation and imagine they're apart of it and in control of it. A few high ranking officials leak serious information and 'undermine' the enemy from within and that's bigged up into this huge intelligence structure.
So that way they can both be incompetent and hyper competent at the same time. Because almost no-one is control and hardly anyone talks to each other they can do huge things but at the same time absolutely bumble around like idiots, and because people aren't on the inside they imagine the thing is a lot more cohesive than it really is.
It's like the way Al-Qaeda can do what they did and yet also have groups of people in the organisation who try to hide a bomb up their arse and have it accidentally detonate (a thing which really happened)
And this fits even better with The Illusive Man, because TIM is a genius but self-blind egotist, like a really evil Julian Assuange. He gives the organisation a face, he's got a lot of personal competence and skill and he can get things done. But he's also much less in control than he can see he is, and he's completely blind to the Reaper indoctrination that's secretly infiltrated and manipulated his organisation since almost inception.
He can get the backers to spend ressurection tech on Shepard, and he's also the only person deluded and obsessed enough with one person that he's willing to spend a world's GDP on making it happen. He likes to thing he controls all his various cells and facilities, but in reality he doesn't. He thinks he's Cerberus and gives them definition, but he's not and Cerberus doesn't really exist as anyone imagines it
