I feel like for most of the OS it was a depending on the writer thing. Misty was recurrently portrayed as incompetent in Kanto, while Brock at least gave Ash a proper challenge when they first met. They're generally portrayed competently during their return cameos after they left.Not to mention Misty has been shown better than Brock during their journey. Especially in Johto. Like in the Pokemon 3 Movie.
As for Kiawe, it's true Brock tanked him in the Kanto trip, but that was early into SM. He kept pace with Ash for most of the series after, and was implied to still give him a difficult battle AFTER he won the league. Then again Sophocles also gave Kiawe a decent challenge despite only doing a crash course in battling.