These rigs are brilliant! You solved the shoulder issue I’ve been having for ages quite nicely. What made you think with using IK to target elbows instead of the Track To constraint? The only complaint I have for the 2-leg rig is that it’s too difficult for the character to pivot from the ball of their feet. It’d be a simple thing to add, though.
It might be nice for you to do a video of your animation workflow in Blender. The controls present are connected in a different manner from most of the rigs I’ve seen, so I’m curious to see how you work when setting keyframes.
What made you do Shinobi as opposed to other characters? (Great modeling too, although there are a number of spots where you could save polys if your target is a game engine.)