The unique configuration of twin coaxial rotors presents both advantages and disadvantages, and the relative weight of each must be considered in light of the aircraft's mission.
I am going to assume the reader has a basic understanding of helicopter controls (cyclic, collective, antitorque).
A coaxial rotor has a much more complex hub than a regular one. It does not just need to invert the rotation direction, it has two sets of cyclic and collective controls that also mimic the effect of a tail rotor.
Since the lower rotor operates in the down-wash of the upper one, its collective pitch needs to be higher to produce the same lift (and thus torque). This is important because a torque imbalance will make the aircraft yaw uncommanded. In fact, this is how yaw is achieved when the pilot steps on the pedals, by letting the rotors generate differential lift, but ensuring their combined total stays the same so the aircraft retains level flight.
The cyclic controls suffer the same effects, that is the least of their problems, see below.
A single rotor in forward flight will assume a slightly conical shape due to the lifting force on the blades. This cone will also be tilted sideways with respect to the direction of flight because the blades see different airspeeds at different points in their cycle. That is to say, the blades will generate more lift while they advance into the direction of flight, and less while they retreat on the opposite side of the helicopter. The higher the forward speed, the larger will this effect be (In fact, there will usually be an area on the rotor where the blades do not produce lift because the airflow over them is reversed: the forward speed of the helicopter is greater than the local speed of the blade).
In a coaxial design, both rotors turn in opposite directions, so their tilts will be in opposite directions too: the clockwise-turning one will tilt right, and the anticlockwise-turning one will tilt left (the actual locations of maximum bending are not quite as clear cut because of blade inertia, but this serves as a basic explanation).
The resultant problem is pretty obvious: the two rotors can intersect each other. This is a Bad Thing(TM) and will lead to all sorts of issues like rapid unplanned dissasembly of the rest of the rotor blades. A pilot in this situation will most likely make use of the conveniently installed ejection mechanism.
The actual forward speed at which self-intersection will occur is not published (to the best of my knowledge), but is probably somewhere above 250km/h. This is only taking forward flight into account... sudden collective or cyclic inputs at high but not critical airpeeds may have the same effect.
The overall effect is that coaxial rotor helicopters are more limited in their flight envelopes than conventional ones. Although this is not a fatal drawback, agility is highly valued for attack helicopters.
This is somewhat speculative, but the lack of a dedicated tail rotor may limit the yaw rate at low forward airspeeds, where most of the yawing toque would come from the tail rotor.
At high speeds the weathervaning effect of the tail becomes dominant anyway, making this a moot point.
Simpler tail design
The lack of a tail rotor frees the helicopter from having to transmit torque to it, thus heavily simplifying the boom design. In fact, some coaxial designs make a particular point of exploiting this advantage, having very short boom, like the Ka-25 and Ka-27, which are navy helicopters meant to fit into the tight confines of a shipboard hangar.
The loss of a tail rotor is not, like many movies would suggest, an unrecoverable situation. A helicopter can be stabilised with sufficient forward speed if it has enough tail volume.
That being said, losing a tail rotor in a combat situation will most likely result in a mission kill. And it's not just the rotor that's vulnerable, the boom contains the transmission rods for it, so any damage along that structure can be potentially fatal.
For a coaxial helicopter,the loss of the tail will lead to some stability issues, but nothing catastrophic.
Whether the advantages of the design outweigh its drawbacks is ultimately a matter of preference and of the fit between the aircraft and its intended mission. The adoption rate of a design is not necessarily an indicative of its merits, especially when military procurement is involved, as there are countless other factors at play.