iClone sure does have a sliding feet problem. But most of the time is does occur if you try to edit a motion clip or blend it with another.
God forbid to delete legs or torso keys somewhere in the middle of the clip. Your character would be all over the place.
It also occurs if you make some unnatural torso movements and then try to edit it to rectify.
Take a look at this demo clip I made. Those are 2 almost identical sitting down motions.
One creates sliding feet, and another almost sliding free.
So in the first sitting down motion there is quite unnatural body movement recorded first.
And even though I sort of rectified it by adding another key in the middle, it did not help at all.
But the second motion was created with sequence of 2 natural movements and the result is much better.
That brings us to your second problem. With no local bones transform, it is easy to make a mess with torso movement (which is a main cause of the sliding feet).
Sometimes you have to go around your character 360 degrees to even notice it. There is no solution to that. You may add your vote here:https://www.reallusion.com/FeedBackTracker/Issue/Please-add-a-Local-Move-for-the-Motion-Layer-Editing
But I sort of doubt it would ever be fixed. And this goes to sliding feet as well.
Those are my personal observations. Someone might add some more causes and solutions.