It can only activate its effect when it's the only monster on the field, but being able to search or Special Summon two monsters from the Deck is extremely useful
Its Blizzard attack costs only three energy and deals a solid 80 damage, making it very efficient
Level 4 is the most common level for a non-tuner monster in the game, so having these level 4 tuners provides an incredibly easy way to synchro summon for level 8
it's amazing when you pull it off, but like trident dragion, it requires you to spend valuable resources on it (debris+debris or debris+drago?) I could see him in a build centered around getting out double delta flyer or the like (2 of them-lv 4
Keep in mind that the Pokmon TCG has grown into one of the largest card games ever printed: with over 15,000 individual cards in circulation, the question of how many pokemon cards are there has no single answer, but knowing which ones matter for Mewtwo specifically is exactly what this section is for