These shields would be better if it were common for cards to have both physical and spell damage. Then they could be somewhat effective most of the time, and extremely effective some times. As it stands they are worthless most of the time, and occasionally autowin. Everyone hates cards like this.
So here is a proposal: Make a new attribute (like flying) called "magical" spread it liberally over all the existing creatures (fire spirit, forest spirit, most dragons, most weapons, etc.). Magical creatures do half physical damage (rounded up) and half spell damage (rounded down) if they hit. So they are still fully blocked by physical shields, but also partially reflected by reflective/mirror shield and emerald shield.