Compared to a grandmaster or computer, they're terrible. For 99% of the population, they're indistinguishable from a master.
Really, 1800 is pretty strong. They have pretty good tactics, a positional understanding that lets them develop sensible plans, and decent endgames. They still make mistakes in all phases of the game, every game, but I think it's fair to say that 1800 is a strong player.
Is the average 1800 USCF strong and steady? Or pretty mistake prone.