some helicopters service ceilings are too low:

notably :

MH9/AH9 : 4050m. instead of more than 5700m (MH6 specs)
UH80 : 3120m. instead of more than 5790m (uh60 specs).
AH99 blackfoot : 3600m. instead of more than 4780m (rah66 Comanche specs)

MI48 Kajman : 4600m. instead of more than 5500m (ka52 specs)

since the game is supposed to takes place 20 years in the futur, meaning a performance increase, it's not logical to have poorer flight performance compared to the previous helicopter generation.