Pros
If you have some form of capability, you will learn very very fast. You get a lot of working time with the founder/CEO who runs the show and will be able to soak up how he works. If you grow as fast as he expects you to, you will soar.
When you leave, you would have grown more than a peer who spent the same amount of time in a different agency.
Cons
Quality levels are high (the CEO's standard), so every single thing that does not equal to 100% of your soul and does not score 100% on your "test" means you are not good enough.
You will not have time to be good enough. You are either capable now or you're not capable ever, and that will be made known to you. No holds barred for how the CEO will communicate that to you.