i can't say i've done any kind of formation flying... other than being towed aloft in a saiplane ;)
but i think that the sim IS harder on some aspects, many of which apply also to flying helicopters...
first, you have NO depth perception... it would make a world of difference if you could perceive yourself getting closer or farther away from your lead plane
also, the joystick will never replicate an actual control stick / yoke, no matter how good... but it does help to have a high precision joystick... i know my flying improved sensibly when i traded over the old MS FFB stick for the X52
plus, your vision is tunneled by the small area of the monitor... a triple head setup helps a lot, but you'll never achieve TOTAL immersion... even completely surrounding yourself with monitors... and that might be bad for your eyesight ;)
so the sim has it's limitations... but at least you can harass the air traffic without fear of being arrested later ;D
Cheers