I’m so confused. Drawbacks of a geared bike? As opposed to what? Flintstoning it?

I must just not be understanding what you mean.

I’ve ridden bikes with no derailer or gears, when you backpedal, you brake.

And I’ve ridden mountain bikes with front and rear gear changing.

I know there are super exotic driveshaft bikes, and electric etc, but besides that, what could you be talking about?