Just made this twice. The above recipe 3:3:2:3 - not for me. Too much presence of the hazelnut and aperitivo. Second time, went with 2:1:1:2 using Drumshanbo, Campari, Frangelico, and Cocchi di Torino. Much more balanced, but still quite beautifully bitter.
I did not enjoy the drink very much, the combination of frangelico and campari ended up tasting like burned hazelnuts. I made a version with aperol instead of campari and bourbon instead of gin and it was very sweet, but the aromas melded more readily. There is a good drink hidden somewhere here.
Use a white Italian bitter instead of the red as it was not as sweet as the red. Ended up with a perfect after-dinner/evening sipper. Starts with the sweet, hazelnut notes and ends with the bitter. Not outstanding, but perfect for a regular evening cocktail.
Actually a very nice slow sipper. Takes off the sharp bitter edge of the Negroni, that sometimes you may not want, and tempers it with the familiar, comforting, rounder Frangelico flavor and mouthfeel. Nothing surprising, but very pleasant.