It's strong in most mono-Black decks, blink decks looking for lands, ramp decks looking for more ways to ramp out lands in the creature base and midrange decks that have loads of card advantage ETB dorks like this to use and abuse
Removal spells before Terminate always had a bunch of extra rules text
Fortunately, thats why we have Kangaskhan or Hypno, and those decks have significant weaknesses that Mewtwo can exploit: Charizard EX takes a long time to set up without Moltres (which Mewtwo EX can OHKO) and Blastoise EX requires a lucky Misty draw to reliably get its Energy
and the last card shows the couple, older, with Gardevoir holding balls of yarn as the woman knits
That means they don't trigger any abilities that occur when a creature attacks or becomes tapped
If your main goal is to make Legacy more affordable and accessible, however, there are other, better, easier ways to do it that dont involve fighting against WotCs seemingly unbreakable oath