Not all lawns are flat. If yours has slopes, dips or uneven terrain, a standard robot mower will likely struggle. Robot lawn mowers for uneven ground & slopes are built with stronger motors, flexible chassis and better wheel grip to handle gradients up to 45% or more.
Slope rating is everything. Always check the maximum incline a model can handle before buying. Brands like Husqvarna, Worx and Segway Navimow all offer models rated for steeper ground.
Look for all-wheel drive and anti-slip tyres. These keep the mower stable on wet grass and uneven surfaces. A low centre of gravity also helps.