What about giving the spies the condition that they have to be placed next to a (non-spying) unit. This would remove some interactions, which are not optimal in my opinion.
You can't just play a spying unit on an empty board, you can't play it directly into weather if there isn't a different unit already in it, you can't play it on an else empty row to utilize it with Crow's Eye.
Beneath the fact that it would constrict spies more to their primary use that you get 1 card advantage in return for a huge point increase for your opponent, it would make sense lore wise, as spies can only spy anything if there is indeed something to spy.
Also, their strength should be increased, but not directly, instead on deploy such that they can't be utilized by cards that benefit from huge strength cards in your hand, like Spear and Wyvern Shield.