I know there's been a thread or two debating penetration with regards to the Aegis (the one I've been trying to read is here), but nothing I've seen has convinced me that a Penetration total is actually relevant when casting Aegis of the Hearth...or any other ward, actually, without totally nerfing Wards altogether. I just can't understand why you would need to get a casting total of 50 for a Level 25 Ward, which supposedly stops any creature (of a particular type) from crossing a entering the circle. Maybe it's just be, but it makes no sense.
Or is that my 5e inexperience talking?