In Pico-8, you can draw sprites from a 128x128 sprite sheet. And you can change the sprite sheet at runtime with a function called sset. Once it's changed, you can just draw sprites like normal! Super simple.
If I wanted to do something in Unity, it would be a lot more complicated; I wouldn't even know where to start.
Pico-8 is fun to work with, and great for rapid development! But it would be harder if I wanted to turn this into a larger game, since I'd have to figure out a way to save the created cards, and Pico-8 has very limited persistent memory. But, working with these limitations is a big part of why its fun to work in! If you're intrigued I say give it a look.