I love Gwent, it's the best mini game in my eyes. But it has it's flaws and could really use some balancing. Spies are vastly superior to muster, this fact alone makes Northern Realms and Nilfgaard top tier by a large margin. There needs to be cap how many hero cards one can play in a deck, and change up boring default 10 power heroes - like reduce power and add an ability. Scoia'tael faction power is weak, easy fix would be to swap it around with leader ability to draw an extra card, then it would more or less match other factions. On top of it Scoia'tael also has the worst medics, they need to get buffed at least to Northern Realms medic level. Monster faction has good faction power and hero abilities. However overall weak card quality and heavy focus on close combat muster makes it, extremely vulnerable to Biting Frost and Villentretenmerth combos with medics and/or decoys. It feels unfinished and weak deck, at the very least change Crones to ranged units and add bonds to low power close combat units. Scoia'tael could perhaps have anti ranged and Monsters anti siege Villentretenmerth, to help close cap with Northern Realms and Nilfgaard. Then there are also really awful plays AI makes sometimes. Worst one was, when I passed and AI was ahead on board and then played Fog to lose a round. After all the shit talking, anyone else expected dude at the Gwent tournament finals to cheat or have overpowered unique cards? Then get disappointed it being just a terrible Monster deck, worse than a random innkeeper playing Nilfgaard deck.
To answer OP, on my 2nd playthrough as Northen Realms I never lost a single game. My worst result was draw vs Scoia'tael when I literally had Villentretenmerth and all 4 spies in the bottom half of my deck.