Well, over the last years battery power and energy storage has improved a lot so I would say it is now totally reasonable that a mower will work fine on batteries. Electric engines have a couple of great characteristics that differs from gas engines:
1. They have higher torque at lower rpm than high rpm, excellent in cars, and bogging down mowers
2. They are much less maintenance, fewer parts, less dirt inside etc
3. Less (local?) emissions. But if you charge from a coal power company, well, less fine. Either way, they smell less!
4. On grid charging is no prob. Off grid, it is still easier to carry fuel than batteries
Things that may be a problem:
A. Battery packs are still expensive, so they may be too small to last a large lawn and feel too expensive to buy an extra battery to be able to "re-fuel"
B. They are a bit of first generation mowers, so some features may not be top notch yet
C. Fast chargers are also expensive
I'm certain that electric will grow quickly into the garden space. The electric's characteristics are great and it addresses the main problem with mowers:
"They don't start unless new or managed by an artist maintenance guy from BITOG, and still require smelly gas cans and sometimes a bodybuilder to pull the cord hard enough!"