Most mowers cope with quite steep slopes (20 – 45%) however read the literature carefully and check if it affects the maximum area they can cover!.
Normally a mower will cope with a much steeper slope in the middle of the garden than at the edge (around the perimeter). Also be aware of sudden changes in height that would cause the mower to get caught e.g. the front of the mower hits the slope as the wheels haven’t got there yet.
Whilst robot mower cope admirably with slopes of varying (quoted specific to each model), it should be noted that weather conditions will also limit these. So be sure not to be overly reliant on a robot cutting close to its maximum quoted incline.