So tried this again as per the recipe, i.e. using Campari (as opposed to using Aperol or Amer Picon to cut bitterness which some found overpowering). As is so often the case, Difford's / Jamie Boudreaux's recipe is perfect unlike my first modification. Campari is definitely the way to go with this cocktail in my view.

10-30-40 is easier. More simple

Just OK, but fit my desire for a cocktail with bourbon and orange juice. I may try this again using apricot liquor in place of maraschino. And I think grapefruit bitters would be a good idea.

Was looking for a drink with bourbon and Campari and came upon this. While making it to Jamie's proportions came up a half ounce short squeezing a rather dry Cara Cara orange and made up the difference with Ocean Spray white grapefruit juice (what a variable beast orange juice can be). Frankly the results tasted weird. Tried a dash of Chartreuse Vegetal Elixer which added complexity but still tasted weird. Then added a dash of Hella ginger bitters and miraculously the weirdness was vanquished.

This was a little too boozy & bitter for some, so I was looking for a way to lengthen & sweeten it. So swapped out half the bourbon for limoncello & upped the orange juice. Found the result a refreshing bittersweet long drink for a summer's evening (recipe below)...

https://www.diffordsguide.com/cocktails/recipe/28758/holy-union

This "Union Club Cocktail" is superior to "Union Club". They seem like very different cocktails to have such similar names.
This one is a fruitier.
Great depth of flavour.

Interesting re comments on Campari being a tad "heavy". Given that, I tried using Amer Picon instead thinking the orange notes would complement the drink. While enjoyable, I think I will opt for the Campari next time as I think the drink would actually benefit from a little more bitterness given the sweetness of the Maraschino and OJ. All in all though, a very nice drink well suited for the summer.

Very balanced. Campari seems a little heavy for me though, and I usually like bitter drinks.

This was just ok. A pleasant drink. I enjoyed it, but not sure I would make it again.

I liked this and gave it 4 stars. Good aperitif. I did find it a bit sweet, but it could be an issue with our oranges as I am in Southern California and they tend to be very sweet. May consider making with grapefruit or a mixture of grapefruit and orange.

This is a very pleasant whiskey cocktail. I wanted to reduce the volume to fit it into a Nick and Nora glass, so I made it 30ml-5ml-5ml-20ml, and I used Wild Turkey Rare Breed (58.4 percent abv) for the bourbon because I have a bottle with a little bit left in the bottom that I'm trying to use up. (You might want to use something lower proof if you're making the full amount in the recipe. The drink is balanced, with the Campari and Maraschino staying in the background to round out the flavor.