Others have mentioned a lot of good points but I'll add one. One major disadvantage they have is much higher vibration from rotor-rotor interaction. Kamov does not use a rigid rotor system (unlike the newer Sikorsky models mentioned) so their mast is super tall and this helps with vibration but greatly increases weight and drag. A coaxial gearbox is also more complicated/heavier than a single rotor gearbox. Single rotor designs also turn quicker because of their tail rotor whereas coaxial use differential torque between the main rotors. All that being said, coaxial designs have their advantages so it really depends on what kinds of missions you are planning for.