[mecanim]: BindSkeleton: cannot find ...? How to fix it?

So I'm starting to learn the animation pipeline for Unity and a bit confused about Mecanim. I was able to get everything working fine for characters but now trying to animate other objects like doors/chairs etc.

First question I have is, should I use the mecanim system or the legacy? The documentation says the legacy will eventually phase out so i assume I should do everything with the new system.

If so how do I set up a generic object properly? I've defined it as generic, chose the root bone but I keep getting a:

 [mecanim]: BindSkeleton: cannot find Transform 'Root'

every time I try to run it.