I tried this last night and rated it 3 stars (good, would make again). If you like Fernet in modest (not super subtle, but not hit-you-on-the-head amounts), you'll like this cocktail. I also subbed Amaro Noninio, but the sugar balance was off. Need to swap some Cynar for more Tequila or maybe dry vermouth.
I initially made this with Fernet Branca and a tequila blanco. I thought this was bit unbalanced using those ingredients, but better when using an alternate, less menthol heavy fernet such as Fernet-Vallet (which made sense given the other Mexican ingredients) or Letherbee Fernet. The type of tequila wasn't specified so I figured the default was a blanco; I thought the oak, vanilla & caramel notes of a quality repo or anejo rounded out the drink better. So if you didn't like this as specified, I'd suggest maybe trying it with an alternate fernet and an aged tequila, which made for a nicely complex, bitter sipper. I also thought a drink with powerful flavors such as this benefited from being served on a large rock or sphere instead of up.