There are many reasons the Control-Flex Coupling provides exceptional value for encoder and other Instrumentation applications:
A large minimum to maximum bore range is available.  The large bore range helps to keep from oversizing the coupling where it is connected to a small diameter encoder shaft on one side, and a larger diameter machine shaft on the other side.
Electrical Isolation is inherent in the design of the Control-Flex Couplings due to the construction of the flex member.  This electrical isolation eliminates any stray electrical current from passing through the coupling and causing bearing issues in the encoder.
Low reaction loads and a high misalignment capacity of the Control-Flex.  The flex member design keeps the reaction loads minimal for the smaller encoder bearings, which provides longer life for the application, even under a misaligned condition.
Zero-backlash.  This is critical for maintaining positioning accuracy in instrumentation applications.
Low inertia.  This provides a fast and accurate response to acceleration and deceleration on the machine.
Clamp-style hubs.  This proven connection method maintains an effective, solid shaft connection during starting, stopping, reversing, etc. on the machine.  This style of hub offers a stronger and more reliable shaft connection than a traditional setscrew design.