There's not enough Dragons yet for Dragon's Mirror, and not enough E Heroes yet for Miracle Fusion
Some of these cards even require you to play multiples in your deck, so let's hope you'll only need one of the binder rather than to take to a tournament
Frankly speaking, fusion spell cards are very effective during combat
Below it sit five Special Illustration Rares worth knowing on sight: Meowth ex SIR #121 with its Team Rocket comic-panel border, Mega Zygarde ex SIR #120 in kaleidoscope artwork, Rosa's Encouragement SIR #123 (the set's top Supporter), Mega Starmie ex SIR #118 with its water-effect holofoil, and the pastel Mega Clefable ex SIR #119
This means it's not possible to cheat creatures into play out of nowhere (which madness can do), but it also means you're granted more flexibility, because you can cast a spell for its mayhem cost at any point during the turn in which it was discarded (rather than having to choose immediately as per madness rules)
However, this creates a turbulent cycle of powerful cards being released then limited or forbidden