The "new" hard and light types of foam will tear when you push the planer too fast. You can install a grit drum or slow down.
The softer creamy foam like the old Ice-9 mowses was nice to shape. You could just jam away.
Tearing can be a nightmare when you're trying to finish off a shape and you gouge the foam. If it's deep enough you won't be able to remove it without making a thinner board.
I try to calculate tearing into the shaping process. Skining and reducing thickness of the blank I just blast away with the planer. When you start to push the planer too fast the sound the planer makes as its cutting will change. As I get closer to the finished shape I try to be more aware of the sound the tool is making. If your pushing too fast the tool will growl as it tears the foam. The deeper the cut the slower you have to go.
Once I know I'm getting into the last 1/8" on the bottom or top I slow way down and make smooth clean passes.