Chainsaws are not that impractical, at least for a makeshift weapon intended mainly to deal with flesh.

I am always curious, reading answers like this. I got my fair share of hands on time with chainsaws and cut a bunch of stuff with it.
And working in a kitchen i have broken down more then one animal with knives and saws.

I would not consider a chainsaw as a weapon at all.

They are easy to tangled in fabric, get stuck, kick back and just over all they act like a horse that does really not want to be ridden.
They need lots of control and thought on the operators part, which is not something you can do "in a fight".

Meat and bones are way harder to cut then people think, granted, i only worked with dead animals that have gone through a bunch of stages of death/decomp which makes the meat "harder" but i do not think a living human is that much different from a dead pig.