Comets lose a measurable amount of mass each time they make a circuit around the sun. The loss of mass is so significant that no comet should last more than 10 trips around the sun – a few thousand years. This is a far shorter life span than allowed by secular estimates.
The uniformitarian assumption directly translates to a younger-earth result.
Consequently, advocates of the old-earth hypothesis invented a rescuing device: an imaginary object, the Oort cloud, which serves as a comet reservoir, occasionally sending fresh comets our way.
There is no observable evidence for the Oort cloud; yet it is fervently believed to exist by people committed to the old-earth hypothesis.