Low voltage requires the use of larger conductors since the current is higher than a comparable 120 V load. For a 20 watt bulb the current is ten times higher, so on a 120 V circuit it only requires ~0.17 amps (20 W/120 V) while a twelve volt 20 W bulb needs ~1.7 amps (20 W/12 V).

Transformers do not have losses that high,and many of the 'electroni' transfomers are acytlly switching power supplies.
They also can be pretty efficient.

Low voltage also allows the use of more rugged filaments that can withstand the vibration from taking things in ans out of the cabinets immediately above the lights.

Puck style lights are typically only 10 or 20 watts.
You just use more of them to cover the counter.
There proximity allows for good lighting.

you can get 60 watt 12 V supplies for pucks that are rather small and easily concealed under the cabinet with the lights.
Each one will power three 20 W pucks.

Incandescent light get hot, LED and florescent not as much.

Just google "low voltage undercabinet lights"; they are quite common in strip and puck forms. But the first thing to decide is what kind of light you like. Go to a lighting store with a kitchen mock-up and see which type of light you think looks best regardless of voltage type.