Combo decks can roughly be divided into two categories. There are the blazingly fast combo decks that are vulnerable to disruption and there are the slower combo decks that are more resilient to disruption.
It's great to have that resilience when playing against fair decks with a ton of discard and counterspells, but in the matchup between the two, it's the speed that wins out, and because combo decks can't sideboard much without hurting their own plan, it's very hard to flip the script in the post-sideboard games.
In Modern, the best example of a resilient combo deck is R/G Through the Breach. As the game goes long, you end up dying to lands, so you absolutely have to establish a clock to beat it. And in a race, the deck kills consistently on turns four and five. But in a format with a lot of fast decks, the slower combo deck can be an awkward place to be.
This list looks to solve that issue with Madcap Experiment. You can consistently cast the card on turn 3 and Platinum Emperion is nearly impossible for decks like Dredge and Burn to answer. Fair decks have things like Path to Exile and Terminate to answer it, but it's still a very powerful play that will win games either by going unanswered or acting as a lightning rod to protect your Primeval Titan.
Of course, nearly everyone will have an answer to Platinum Emperion after sideboarding, but forcing them to bring in a narrow answer generally favors the person on the threat side of the equation. You also have the option of next-leveling them by bringing the package out and leaving them with dead cards.
The elephant in the room here is Infect. It's among the deck's worst matchups and Platinum Emperion doesn't stop poison counters from accumulating. The Platinum Angels in the sideboard are a nice swap for the matchup, but only if your opponent isn't expecting Madcap Experiment and won't preemptively bring in Nature's Claims and the like.
But you can't have everything in Modern. Helping in a number of matchups still makes the plan very interesting, given that it doesn't take up too many slots. Simian Spirit Guide is the main cut and the main purpose of that card was to help recoup the speed you needed in combo matchups, which hopefully this plan will make obsolete. The potential gains are certainly worthwhile.