What is "Unplayable"?

It quite literally means that the game cannot be played - most often due to crashes and other bugs.

With that said, "unplayable" is also used, by some, to express a feeling of the game not being rewarding in a sense of the game trying to make the gameplay miserable for the player. So when they say, "it's unplayable" - what they mean is "it's unfair".

Imagine playing a FPS game where AI runs faster than you, has more ammo than you, has more HP than you, and has an aimbot - not very fair is it? Or playing an RTS game where AI build time is faster than yours the harder the difficulty, and also they see through fog of war - not very fair either. This AI design make the gameplay miserable to the human player, resulting in no desire to play the game and eventually regret buying it to begin with.

I think that the AI needs more balancing in this game. I don't mind a "cheat" advantage that they get to make the gameplay more engaging. Because lowering the difficulty is not a great solution, where AI in lower difficulties is completely underpowered in one aspect of racing and is still overpowered in the other aspect of racing. It's almost as if the difficulty slider affects the AI's pace only in corner entry and mid corner speeds, but doesn't address the exit and straight line speed as much.