Controversial opinion... but I think any card that generates card from the ether and puts it in hand should be revealed to the opponent.
That's generally how it works in other card games. Like if I pull something from the graveyard/deck, I have to actually show the card to the opponent. Now of course in TCGs you have the element of cheating where if you didn't show the card to your opponent then you could just pull anything from your deck/graveyard.
Well in HS you don't need to show cards to your opponent because the game does it without any ways to cheat. However, it takes away a fundamental skill from the game which is playing around certain cards in the opponent's hand. Powerful card effects that pull something out tend to be balanced with the fact that you have to show it to your opponent so they can prepare for it in advance.
This includes Discover cards too. I know people love Discovers and what not but having to play around random cards that you have no idea about because they were pulled from the ether isn't exactly fun or even fair. This isn't even a taboo concept in HS... a card like King's Elekk when it wins the joust shows the card that is added to the hand. Does it make the card fundamentally weak because you are giving away information to your opponent? Sure but it still sees play.