We can only hope that the money invested in Leaf Tickets will not be lost
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As most players have the bad habit of casting everything on their first main phase, it's incredibly likely they'll move to combat and realise they've splurged all their mana and can no longer afford to attack you
The oracle text really helps here since it reads At the beginning of the end step, destroy all untapped creatures that didnt attack this turn, except for creatures that couldnt attack. If we look back at the Arbiter we see that there's no restriction on which creature can attack, so summoning sickness aside on my turn, it is likely that all but one of my creatures will get destroyed
Sunscourge Champion is fine with Panharmonicon but worse than it looks because generally you want to cast Sunscourge Champion on Turn 3, which happens to be the turn before Panharmonicon hits the battlefield