Give the drawbacks to use period of a pendulum as a time standard?

Question

Give the drawbacks to use period of a pendulum as a time standard?

Answer 1

Air resistance is one reason.

Also a pendulum is not-strictly linear.  To second order, the period DOES, in fact, depend on amplitude (insofar as sin(theta) is not exactly equal to theta).

Answer 2

Adding points to other answers:

Pendulums are made of metal whose length change with temperature. In summer, the length increases increasing its time period and pendulum oscillates slowly and vice versa in winter. In old times when we used to have pendulum clocks, I remember I had to adjuct the length of the pendulum with change in season by trial and error. Present day quartz clocks have no such problem.

Answer 3

If a bunch of large meteors were to crash into the earth, changing the mass of the planet, and thus changing the gravitational attraction from the sun, would have a slight affect on the acceleration of the pendulum.