I tried to search around for this but couldn't find any answer. Half the time my Yghern gets destroyed when summoned and the other half of the time it gets deployed and armor created as usual. Is there something I'm missing or is this just buggy and annoying?
Because if summoned (Ihuarraquax, Golyat Deathwish) he doesn't trigger his deploy ability (Deploy: Gain Armor equal to the number of cards in your hand.) and therefore doesn't get any armor which triggers his other Card effect (If this unit has no Armor, destroy self.) leading to destroying himself which also happens if you use the Imposter Leader Skill or Dudu on a opponents Yghern to survive he needs to be played either from Hand, with a Tudor Card like Oneiromancy or with draw and Play Card like Dandelion or Cantarella with at least 1 Card left in hand.
Edit: the only possible way to make him survive while getting summoned is to put Armor on him with Iron Falcon Troubadour in Round 1 and to put him back into you're Deck in Round 2, which usually no one does because it doesn't make much sense putting Yghern back in the Deck in round 2.
Deploying is not the same as summoning in Gwent, because summoning doesn't trigger deploy-effects.
(Like Skellige's Jutta an Dimun doesn't get her health cut in half when summoned by Sigrdrifa's Rite.)