In the view of this truly enormous potential space, thorough playtesting is obviously impossible. The designer and playtesters can't test every possible spoiler deck, and every new idea has a reasonable chance of being a spoiler deck. They continue to accept the increasing returns to scale. Then they playtest and tweak the game to make the main strategies approximately equal. The result is a game that appears to work at first glance, but doesn't stand up to detailed examination.
Most themes turn out unviable, no matter how clever the player is at design.
Second, although relatively small, these spaces are still enormous. So inevitably, spoiler decks get missed. Any game which requires limits to work necessarily carries significant flaws. Initially, the game can look really cool. After a while, though, players find their themes are very strongly constrained and the good decks are all from a repetitive set of spoiler decks.