It's not the amount of points schirru can get, it's about it being too easy to achieve and only possible to play around it if your deck allows it, and the sheer fact that Schirru alone can make alot of different decks unviable.
If you play a specific thing and your opponent runs a card that hard counters it that's bad luck. If there's a card that hard counters several decks all with minimal effort then it should be rethought.
Interesting... I can't think of a single deck I've played or faced in recent memory where a single card or leader is an auto-loss. So perhaps we have different perspectives. Sure, it can be difficult to win against certain match-ups depending on the coin result, draws, last say, etc. This is the nature of the game though. Some decks simply have a hard time against other decks or situations. Gwent has always been this way.
I will say if you build a deck with a lot of units, most of which have similar sized bodies, when you have no way to alter them and think you're going to fair well in a long R3 vs a Schirru deck it's no surprise our perspectives differ. A long R3 with this type of deck against Schirru is called playing into Schirru. It's no different from a long round against most ST builds, or stuff like Dragons Dream based decks. It's not much different from what decks hinging on last say can do if you give them what they want. Easy fix, don't give them what they want. Play into R2 to force a shorter R3. Yes, this risks CA. Regardless, sometimes you need to push R1 and/or bleed R2, despite the possibility of lost CA, to avoid getting burned. Identifying when to do so and when not to is one of the harder aspects of the game to grasp.
In regards to countering several decks... Again, this isn't unique to Schirru. There are tons of cards capable of hitting specific decks quite severely. It's sort of why they are so frequently included.
Lastly, "viable" decks have always had a basis in the existing meta-game. You cannot simply throw cards together, pretend there isn't any established meta, and expect to win consistently. Your deck must account for it. Schirru happens to be a large part of it for various reasons. Adjust around it. Calling for nerfs because it beats you is unreasonable. If a deck cannot deal with staple cards in the meta it sounds like it's time to go back to the drawing board.