Why Asyncronous designs are not preffered ??

With all of the potential advantages of asynchronous circuits, one might wonder why synchronous systems predominate. The reason is that asynchronous circuits have several problems as well: Asynchronous circuits are more difficult to design in an ad hoc fashion than synchronous circuits. In a synchronous system, a designer can simply define the combinational logic necessary to compute the given function, and surround it with latches. 

By setting the clock rate to a long enough period, all worries about hazards (undesired signal transitions) and the dynamic state of the circuit are removed. In contrast, designers of asynchronous systems must pay a great deal of attention to the dynamic state of the circuit. Hazards must also be removed from the circuit, or not introduced in the first place, to avoid incorrect results.

The ordering of operations, which was fixed by the placement of latches in a synchronous system, must be carefully ensured by the asynchronous control logic. For complex systems, these issues become too difficult to handle by hand. 

 Unfortunately, asynchronous circuits in general cannot leverage off of existing CAD tools and implementation alternatives for synchronous systems.

 For example, some asynchronous methodologies allow only algebraic manipulations (associative, commutative, and De-Morgan's Law) for logic decomposition, and many do not even allow these. 

Placement, routing, partitioning, logic synthesis, and most other CAD tools either need modifications for asynchronous circuits, or are not applicable at all. 

Finally, even though most of the advantages of asynchronous circuits are towards higher performance, it isn't clear that asynchronous circuits are actually any faster in practice. Asynchronous circuits generally require extra time due to their signaling policies, thus increasing average-case delay