- In the Mash-up format, both players build a deck made up of any cards they want, but the two decks are shuffled together into one big deck that both players draw from.
- In the Set format, players build their own separate decks but have to choose cards from a given "set" of cards curated by a player (and ostensibly the cards within a set should be roughly balanced).
- In the draft formats (Set Draft and Everything Draft), both players are drafting from the same pool of crads.
This way, overpowered (and underpowered) cards can still be created by players, but at least in these game formats, both players would be equally likely to draw said overpowered cards.
If you're interested in the development process behind Untrusted, I wrote a bit about it back in 2014: https://alex.nisnevich.com/blog/2014/10/17/some_words_on_unt...
And if you liked Untrusted, you might like my latest game, Wordbots. It's a very different take on the "players can modify the game code" concept: https://wordbots.io/
Alex