Whole IK Retargeter seems a bit buggy. When you wait to compression of retargeted animation it takes insanely long and no progress bar at all (short 9 sec animation).
And one thing i found really silly is, that you can’t use pose asset with already created default pose and you must make it (again) by hand. Some load pose asset (as is “Modify Pose” in old retarget options inside skeleton) would be really nice. It’s really annoying fiddling pose by hand again and again and again… Mainly if you can make it by script, save it as pose and then load it in sec. Oh wait, that’s no longer possible 
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I’ve tried set default pose inside skeleton, but i find out, that this settings are ignored by new IK Retargeter.
EDIT: Also seems that get fingers retargeting right with this with new pose setting in Retargeter it’s really tricky.