For the written test. there's parts where you would be shown a helicopter for 100 milliseconds then have to remember the configuration, number of rotors, ordinance... Or you see a tank for a split second and have to correctly identify the barrel measurements and other little details.
The stealth mission was difficult too. I managed to be a medic and a ranger but not special forces.
The special forces test was nuts, was playing on a friend's account at the time but it boiled down to just crawling through the lowest point along the entire path. Literally the entire mission you're in a drainage or small creek just crawling and going stealth. I can't remember if you eventually fight or do anything, I just remember the two hours of crawling on the ground to go undetected.
After I got the SF certification you could play this map called Hospital where you're extracting a VIP while an insurgent team is trying to kill him. So much fucking fun. I loved this game. Yvan eht nioj