This is really an engineering question, imo, but I like applied physics.
There is an alternative to reaction wheels, that is thrusters at the rear which allow the machine to get closer to trees, powerlines and general operate as safely as possible in confined space.
Also, many models of helicopters use ducted rear rotors, such as that shown below.
To counter the weight of the machine, and the torque of the main rotor of the helicopter, the reaction wheel, as I'm sure you know, would either have to be very heavy, or have a serious angular velocity, to achieve sufficient angular momentum and perform a useful stability restoring role.
The acid test for helicopter design is, in my opinion, do the military incorporate the ideas? If they don't, then there is probably a drawback to prevent further research.
EDIT The other answers regarding acceleration of the reaction wheel pretty much explain the line above, it's not just a drawback, it's impossible to implement. I should looked more into the mechanics of reaction wheels before answering. C'est la vie. END EDIT