Looking back, the CFI portion of ATP's program is hard, but its only 2 weeks and of that its really only 1 week of hard studying. 121 ground school I thought was ten times harder. You're learning complicated systems on Turbine aircraft, no study guides made for you, no "gouge" like you get at the CFI program and you're not learning stuff you *should* already know. In addition to the systems, you're learning a whole new set of regs, operating procedures, flows, callouts, memory items etc. There is no comparison in my mind.
The CFI was tough, don't get me wrong, but I didn't kill myself over it. Just about everything you are required to know to pass the CFI is pretty much everything you should know as a Commercial pilot, minus the certification regs as it relates to endorsements etc. I think the CFI intial is the hardest checkride you'll take, for the fact that you are expected to perform at a CFI level, this includes being able to teach it. The sim ride I took on the B1900 was pretty straight forward and you knew what was coming.
Don't sweat the CFI though. Its easy to get swamped in the beginning because they present a ton of information and you feel like you don't know any of it, but you do. Its burried in your mind somewhere and once you start to get that part of the brain working again, you'll recall quite a bit. 121 ground school, thats something not to take lightly. You aren't a customer anymore, you're an employee. So they expect you to do well and they certainly don't give you any handouts. I actually had to keep rereading our CFM to understand some things.