since there is only 1 slot available for them they have to be more powerful than normal cards.
+1, same about weapons.
+2. Also, most elements have something that deals with shields, either by destroying them outright or by allowing creatures to bypass them, or doing direct damage that bypasses them, or by attempting to prevent the other player from being able to play them, etc.
They pretty much always remain an issue, but it's an issue that both sides always have access to, and both sides usually have the opportunity to include cards in their deck that would solve (or at least mitigate) the issue.