I've heard the Normandy hedgerows described as a tangled mass of tree roots, dirt and rocks which were on average 10 feet wide and 10 feet high in places. I don't think a flail would make much headway through something like that. I'm even kind of surprised that the hedgehog managed to be effective. I'll bet they couldn't just break through anywhere but had to pick a weak spot and hope there wasn't an 88mm lined up on it.
http://www.lonesentry.com/normandy_lessons/index.html