These are the cards in question. It seems Slowking booked some succes recently. But I'm not sure why players don't use Crawdaunt instead for energy denial. I listed some pros and cons for both of them.
Crawdaunt discards the energy attached to the active Pokémon without a coin flip. It's a certain hit. The energy is no longer in play. There may be moments where Royal Flash can only move energy to other potential threads. Crawdaunt can be searched with Level Ball. Devolution Spray let's you use Unruly Claw multiple times. Corphish has a disruptive attack that let's you discard a card from your opponent's hand.
Royal Flash can be used every turn. You also do not have to worry about energy being retrieved with Mega Turbo or certain attacks. Special Energy that is type specific can be removed. This way you can effectively remove DDE from a Giratina EX. Slowking's entire line can be searched with Dive Ball. Slowpoke has a semi useful attack that let's you search a Pokémon.