How much drawback do deckbuilding restrictions really give to a powerful card?

If we look at card game history, powerful cards with deckbuilding restrictions have a tendency dominate and even break the meta again and again.

In Hearthstone, Prince Keleseth was widely rated as trash during his reveal because of his extreme deckbuilding restriction. Once released, Keleseth Rogue quickly rose to Tier 1. Baku and Genn, two other cards with demanding deckbuilding restrictions, were so dominant that they became the first two cards to ever be banned from Hearthstone Standard. Recently, Drek'Thar, yet another powerful card with deckbuilding restriction, broke the meta and had to be nerfed to the ground.

In Magic, the Companion creatures came with a variety of deckbuilding restrictions, and many of them quickly dominated the meta. Magic usually only bans cards and doesn't nerf them, because it's weird to have rules that are different from the printed cards. But the Companion mechanic was so broken that they actually nerfed the Companion mechanic, so it no longer does what it says on the card. The alternative would have been to ban most Companion creatures. A few of those cards continue to be top tier even after the nerf.

I'll also briefly mention Evenhanded Golem in Eternal, which is another powerful card with deckbuilding restriction that had to be nerfed three separate times before finally losing dominance.

Back to Gwent, two of the most maligned cards are the Golden Nekker and Renfri, both are extremely powerful cards that are balanced by deckbuilding restrictions. Golden Nekker became the primary archetype in multiple factions. Even though the Golden Nekker archetype died down after the Aerondight nerf, the fact that the card exists means a lot of cards can only see print at 10p from now on, when they would have been balanced at 9p without GN. It remains to be seen how much Renfri is overtuned, I think most can agree that she's at least a little bit overtuned, and she will likely see play in multiple factions going forward unless something changes.

All in all, my takeaway from all of this is that, deckbuilding restrictions are usually not the drawback people think they are, and they are really difficult to balance. Nearly every single time when card game designers think a restriction justifies an extremely powerful effect, they were proven at least slightly wrong in the end… and more often than not, they got it really wrong.