Is there an upper limit on gear ratios?

I have a stepper motor purchased for another project that never came to be. I'm thinking of using this one which has a 1.8 degree per step motor but a 100:1 gearbox resulting in a 0.018 degree per step (give or take) but lots of torque. I've only recently started using Astroberry but am keen to create some additional fine focus control as I've not been happy with some of the results I've had with a set and forget approach to focusing.

My scope (Skywatcher Classic 200p) has no fine focus knob, only a coarse knob so I wanted to have more steps to better control the movement and I plan to couple the motor directly to the coarse focus shaft with a coupler and will dust off the 3d printer to cobble together a housing for the motor...

Anyone else with experience of high gear ratios and any limitations or caveats to be aware of?

Thanks in advance,