Yep, rigidBody node is a bit wired. It needs a physics asset to be set up. The bodies in that phat need to be at which simulation state? All set to kinematic except for those you want to move (then to default)? Or all at default and those moving set to simulate. If its the latter then whats the point? Those would simulate right from the start even without that rigidBody node in your animBP.
Also if you use the rigidiBody node then the things wont be able to collide with external objects - thats a bummer, why is that?
Whats the recommended workflow? Do you make a separate physics asset that ONLY contains the things you want to simulate via rigidbody and keep the other stuff in its own regular phat? Then set the override phat in the rigidBody node and do that? Thats cool, but then still you get no collision, is that feature planned? Or has collision been taken out for performance reasons? If so it gains no extra benefits for those that already have a neat animDynamics setup going. I love the amount of control you have with that, you can dynamically tweak different things like spring and damp values, and move affected bones pre and the animDynamics still do exactly what you want, while the rigidBody completely overrides any bone transforms you might be having from things such as customization. So yeah, there is a bit more things to consider when evaluating animDynamics vs rigidBody so far. I really hope that you guys will keep improving rigidBody and give it the amount of flexibility and control that animDynamics had, then, and ONLY then it really going to be completely superior.