Hi!
Was just working on ragdoll yesterday.
In my case i had no problems, but i do indeed use a phys body on the root.
Just add phys body to the root, then constraint to the pelvis. Make it kinematic-disable collision
And set all limits to free. This way you can keep it on the asset without issues.
Also momentum seems to be lost in switching to ragdoll so i just keep track of velocity of certain bones then when switching to ragdoll i apply the saved velocity as impulse to them. 