Why aren't contra-rotating turboprop engines used on aircraft more? They can produce an incredible amount of power, can increase efficiency by 6%-16%, and can go incredibly fast. Sure, they are a bit noisier, and the addition of the secondary propeller results in additional weight and complexity, but these don't necessarily outweigh the benefits. Take one of the only examples of these engines:

Kusnetsov NK-12MA Engine, used on both the Tupolev Tu-95 Bear and the Antonov An-22.
>15000 ESHP x 4 engines
>weighed 6393 lbs (2900 kg)
>sfc= 0.360 lb/shp-hr*
>turbine inlet temp: 1250K (977°C)
>single spool axial flow compressor (14 stage)
>12-chamber cannular combustor
>5-stage turbine
*Specific Fuel Consumption
For comparison sake, the specific fuel consumption for a common modern turboprop:
>Pratt & Whitney PT6T-6B
sfc= 0.591 lb/shp-hr

The Kusnetsov NK12MA is old technology, with the newer model (-M) designed in 1953. For an engine designed in the 50s, it is incredible! Why isn't the contra-rotating turboprop used more often? Surely with modern technology the engine can be modified for even greater efficiency, considering modern turbines can withstand up to 1200°C+.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19830002859.pdf
https://en.wikipedia.org/wiki/Kuznetsov_NK-12
http://www.jet-engine.net/civtsspec.html
https://en.wikipedia.org/wiki/Pratt_%26_Whitney_Canada_PT6