what are the draw backs in using simple pendulum as a time standard?

 An assumption is made in that a simple pendulum undergoes simple

 harmonic motion for small deflection angles (θ) by assuming that sin[θ] = θ.

 As θ slowly decreases over time (due to friction) this results in a time error

 because the clock can only be calibrated for one θ value.

 Simple pendulums need to be calibrated since the period of oscillation

 depends on "g" which in turn depends on where you are on earth.

 Simple pendulums are awkward in that if you move it you affect its motion.

The length of the pendulum varies with temperature, so that means you get an error that varies with temperature. This can be compensated for, but not easily.

The rate varies with the value of g, nominally 9.8 m/s², but that value varies by a few tents of a percent as you move around on the earth or change altitude.

Pendulum clock cannot be use by something in motion, such as a ship.

Large and bulky.

Needs a mechanism to keep it moving.

and, yes, from other answer, rate is very dependent on amplitude of swing.

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