yup those chainsaws are definitely gonna be a useful melee weapon

Only one on one. They are heavy and require both hands, leaving no room for defence, and leaving you open to attack from your blindspots. Also, a chainsaw will remove approx 4lbs of flesh per second, the splatter and chance of bio transfer at close range would be enourmous.
You are also in survival conditions- a chainsaw requires a fuel source, and its weight makes carrying it energy inefficient.

Looking at it in reality, the chainsaw is a poor choice of zombie defence.
Don't let the door hit ya' where the good lord split ya'