One of the downsides of cordless unfortunately, a balance between power output and operational times. I'm sure I saw somewhere that a typical 40v cordless mower is rated at around 500W. An electric mower will be around 1,300W and a petrol mower will be 2,000+W. For a small size lawn it might be ideal, but I wouldn't expect the same performance and cut quality to that of an electric or petrol equivalent. Having said that, I'll shortly be purchasing a cordless mower myself for testing.
In general, for a rotary mower the faster and more powerful engine the better the cut. A better and cleaner cut will ultimately give you a nicer looking and healthier lawn.