Today, I would like to talk about the concept of playing two playsets of the Union monsters - A - Assault Core, B - Buster Drake and C - Crush Wyvern, and the underlying principle regarding it.
Some combo variants of the ABC deck includes Karakuri, Brilliant Fusion engine or even both together. In Karakuri, or synchro-spams , you might wanna play three copies of B and C, because the search and spawn is needed to flood the field for synchro materials. Out of all three, B's search is probably the most triggered and used effect.
The idea of running only 2 playsets is that you can filter your deck fast. Due to Union Hanger, you will be fetching cards from the deck, either for equipping or searching.
Hopefully you won't draw into your Union monsters, as you would rather draw into your field spell or other cards in your deck, such as Vanity's Emptiness, Anti-Spell Fragrance. So with only 2 playsets, your deck will be out of Union monsters fast. Which is a good thing.
I repeat: a good thing.
This allows you to increase your chances of drawing, during the Draw Phases or via cards like Upstart Goblin, your floodgates or cards that can respond to your opponent's threats or attempts to thwart your plans - Solemn Strike, Maxx "C".
That is of course just my opinion. I think that combo ABC builds like Karakuri/Brilliant and even Galaxy Soldier build is a rush game. So late game wise, combo pieces aren't that useful. If you argue that you can combo in late game, chances are it is already too late.
I previously played my Cyber ABC deck, and only ran 2 playsets as well. Here is the solo article about that deck, which recently got some TCG players interested as well.
In short, two playsets of A, B and C is fine, as long you aren't playing standard Rank4 build has been topping in the OCG. For a Rank4 centric deck like that, you require resources, and playing Pot of Desires only increases your risk of banishing your union parts. In fact, it is expected that one or two will be banished, so if you play Desires, you must definitely play 3 playsets of A, B and C.
Limiting yourself to only two playsets also helps with deck constraint issues, allowing you to maybe include more tech cards or floodgates options and maybe even hand traps or luxury choices like Burial from the Different Dimension.