The problem is getting only a few undercut.
I have done it with only 300 or so to go through and had the cancel select 150 of them. A few are for odd "canceling to repost" reasons or something like that.
I just did a cancel run with about 216 items posted. It canceled 7 of them. The cancel bar only got halfway across the screen in this case as seems to always be the case for me.
This is not a problem that stops me, just annoying and I have to watch carefully to make sure it is really done.