Houdini is effectively a no-op Garden backend portable to esoteric platforms like Darwin and Windows.
For a Linux Garden backend, please use Garden Linux instead.
Houdini makes no attempt to isolate containers from each other. It puts them in a working directory, and hopes for the best.
On Windows, it will at least use Job Objects to ensure processes are fully cleaned up. On OS X, there are basically no good ways to do this, so it doesn't bother.