See above reply...I actually disagree with your whole point.
First of all, why does everyone seems to only think about balance to establish whether or not a card is a problem? Balance is important but it's not everything.
The problem of Defenders isn't balance but design. The reason why CDPR put them in the game is because Gwent's control is too aggressive...So instead of redesigning the game to make it more playable they went for the lazy option: Make a card type that kills control.
Here's the thing though, Gwent (and every card game, for that matter) is about playing with another player, interraction is everything, this is where the fun of the game comes.
Damaging this unit so I can work towards removing it, locking that one to slows down your value, boosting this ally to put it out of range of a specific removal....That's what the game is about. If you remove that aspect, the game become boring as hell.
And that's specifically the problem with defenders, they basically read "Draw your purify or stop playing the game" which is an incredibly poor choice of effect if you ask me.
Like, good lord, Gwent already lacks of interraction between players (compared to a lot of other CCG) but now they straight up make a card type that removes it altogether, what a brilliant idea.
Also, it's funny how CDPR clearly realized that overusing immune was a bad idea (which ended up by redesigning multiple card, Eredin's effect, Avallach ect) just to make a card type that virtually give immune to an entire row until it leaves the board...