Card adventage vs. tempo
Card adventage vs. tempo aka "Why everybody plays spies, but when I use them I lose terribly?"
In gwent there exist a concept of card adventage that is similar to all other card games - and is quite natural. You gain adventage if you use less cards than opponent o achieve something, because you have more cards left to do your stuff. If you play 3 Wild Hunt Riders, and buff them twice with Thunderbolt Potion, while you opponent plays 3 Wild Hunt Riders, and buffs each with Swallow Potion you rached the same board state. But you used 3 cards, and oppoent used 4 cards, and you can probably win that round. If you Scorch opponents Eradin, you just exchanged 1 card for 1 card, which is OK. But if Scorch oppoents Eredin, that is buffed with Swallow Potion you killed 2 opponets cards with just one, gaining an adventage.
Spies and decoy cards are terrible in this mindset. Imagine you both play decks consisting only of Redanian Knights. Would you add Prince Stennis to this deck? The answer is no. Imagine last card on your hand is Prince Stennis. You play him, you draw Redanian Knight, you play Redanian Knight. You used up 1 card and gained 0 adventage in strength. You would have wanted just another Knight instead and gain some adventage. Same is true for Decoy - by Decoying your Redanian Knight, and replaying him you just lose 1 card and gain 0 strength. You would have just wanted another Redanian Knight!. (Decoy can be usted with powerful cards like medics, or as healing etc. to negate this effect and even generate card adventage. Spies can't)
So why everybody plays those cards? Because they generate another important resource - tempo. In fact they sacrifice card adventage in order to gain some tempo. And what is tempo? Tempo is the ability to play card last. You have probably experienced this before - you have strong melee lane, and your last card on hand is First Light. You play it, fetch just another Redanian Knight, the opponent plays Biting Frost as his last card stealing the game. There are so many powerful combinations, that can be used, but the weather is the easiest and the most powerful example.
Spies and decoy (and Scoia'teal ability, and many other cards and passing at the right moment) can generate tempo. Basically each turn you are forced to use a card from your hand, but by playing Prince Stennis you cheat it. You use card, that replaces itself, therefore swinging the privelage of playing last card one step closer to you. It does not always work (eg. opponent already passed) but when used correctly you will be sure to have the adventage.
This is what I dislike in Gwent ATM - there are too many ways to just swing the entire board that you cannot answer if you dont have moves left (aard + fog nerf plz). There are decks that rely solely on tempo and win despite generating huge amounts of card disadventage - they can wipe everything you do in a few last moves.
But if your deck cannot utilize tempo adventage to pull off something sick (or prevent opponent from doing so) then the chances are playing spies will hurt you more than it is worth it. And crafting Avallac'h as first legendary is a terrible idea.