Piston extension and retraction timings are not consistent.

The pistons each time take different delays to be extended and retracted, causing things to work unpredictably. Other words: Piston behaviours are not consistent.

The pistons in game have different behaviours each time, which causes the redstone contraptions to work for some times, but not every time.

Example of problem:

Try making a horizontal double piston extender with following delays:

No delay, 5 tick delay, 2 tick delay

(in order from block to last piston.)

You'll notice that the last piston sometimes pulls the first piston sometimes but sometimes doesn't.. 

Due to incorrect timing of pistons, one has to take more sufficient delay between two piston operations, so that even if the pistons extend/retract with some delay then even the circuit works fine.

Consistency is most important for a thing like redstone to have.