Pros
You get to see a lot of ways things won't work. Seriously: the work-life balance isn't the worst I've experienced. Old man doesn't like paying overtime, so he will tell you to go home when the bell rings. Unless he's in the middle of chewing you out for doing what he told you to do. Then you're stuck for a while.
Cons
You will never get a proper raise, no matter what you're promised, and if you show competence at anything, that will become your job, even if you hate it. And you will still have to do your original job. There is no room for growth in a direction you want to go. You only get to grow if the old man thinks it's his idea, and it'll make you miserable. Never saw that man smile more than when he'd made an employee cry (saw it happen several times). Don't expect any kind of actual reward for innovation or efficiency. It is usually punished.