This mechanic may have started out as a fusing of two specific monsters to make a specific other monster, but it plays out very differently in modern Yu-Gi-Oh
Most decks will not play Oranguru ASR 133
C Common Forbidden Forbidden Heavy Storm X SPELL Destroy all Spell and Trap Cards on the field
Official pull rates are not published by The Pokmon Company
Besides it, we have its predecessor, Mew-EX DRX 46 which is not only restricted to copying attacks from the Pokemon in play, but also opponent's Pokemon (as long as it fulfills the necessary energy requirement) The cards that could cause trouble to it are: Path to the Peak CRE 148, Power Plant UNB 183, Silent Lab PRC 140 and Mimikyu CEC 97 (this last one specially to Mewtwo & Mew Tag Team)
It's a great system, which means that players will be constantly checking the app, and always able to find the cards that they seek