I'm forced to agree with the several people who have pointed out that the Mystic Echo Harmony deck is virtually unstoppable. It's the only deck I just can't beat no matter what deck I'm using, and it's the only deck I now refuse to GG; at least I have around a 50% of victory against all of the various Nilfgaard decks, even Soldier Ball. If I only cared about victory and reaching Pro Rank, it's what I would be playing. Also, Igni and Yrden only really counter bad players; good players avoid lining up everything, even it makes Oak worse. Regardless, I'm not spending the scraps to craft tech legendaries that may no longer be playable next month.
I tend to agree with this. Igni isn't really a problem, as the deck allows you to adjust the value of one of the dryads, if your opponent hasn't already themselves, with cards like Minerx2, Barnabus, heck at a push you could even use Pavko, so really it is unlikely igni will take out more than one engine.a
Regardig Yrden, this is more dangerous, but I personally tend to spread my boost engines across both rows, also the flexibility of Great Oak allows you to go to face rather than boost yourself. Add in Malena where you can adjust the composition of your rows on the fly, and it's not a massive concern for me. But I can definitely see it being a game winner last card if things were tight.
Another example, this deck is so strong, I've survived a near total row wipe from Regis toward the end of R3 and still pulled out the win.
Though, I have noticed that this deck has the most problems with SK druids due to their Scenario abuse.
Regarding receiving GGs I get them nearly every game when trying out Harmony. I almost never got them when trying out Soldier's Ball (with good reason I might add, that deck is pure poison)