[Sledgehammer427]

Okay, where to start.
Lurker made the Run silent, Run Deep campaign mod for the pacific theater and then he shifted over to the Atlantic to make Operation Monsun (of course it just started as the trip from lorient to the base in penang, but then it just kept expanding.
What lurker has always done is extensively researched and then implemented into the game the way he likes it, he's a stickler for the details, so as to your question of realism, it's very realistic.
The campaign difficulty is as you would expect, from late 1942 on it's more of a fight for survival than a free hunt. I suggest you start early and get used to how the campaign works before trying to do the Operation Monsun run.
There are no mod limitations, depending on what you are asking, it's a full 39-45 campaign, with training flotillas and training missions, and the whole shebang. there are a slew of new user missions, historic to the detail, it's not just to see if you can accomplish the mission, it's to see if you could actually do what the commander did or better. and that's the point behind the campaign. at least, that's what it is to me.
give it a try, I guarantee you won't be disappointed.