In short, Mewtwo transforms rather than evolves, and its transformations are among the most powerful in the entire franchise
For Gandalf specifically, it helps that he was a powerful wizard, which opens up itself to lots of fun card designs, and that his character evolves significantly throughout The Lord of the Rings
Earthen Vessel is great in every deck in the format, and Professor Sada's Vitality is amazing in every Ancient strategy
Unless you go up against Razor Swine and Phyrexian Crusader, first strike damage is a great way to stop infect creatures, and any combination of first strike creatures can do it Archetype of Courage, Karlach, Fury of Avernus, Abandon Reason, and Sword of Vengeance are great tools to help in combat against infect
Konami generally tries to maintain competitive balance with the range of new cards it releases - but with a game as complex and high-powered as Yugioh, cards will always emerge that 'warp the meta' and generate bad player feedback
:) So it often comes out, sits as an early deterrent blocker, and mostly gets ignored until the battlefield starts getting a little bigger and then if I need them, constructs start appearing