They're both unpleasant people with lots of insecurities but they're different, and unfortunately the combination of their personalities was deadly.
Meghan seems to have researched Harry and effectively stalked him, and he was just ripe for the picking, having lost 2 gfs who shied away from royal life.
She then mined his tiny brain for information about all his resentments, insecurities, grudges etc and knew exactly how to press all the buttons. She particularly pressed the 'I lost my wonderful mother, Diana' button in all kinds of creepy ways. Harry was just too stupid to see what was going on. He ignored a whole beachful of red flags.
I'm sure she had Katherine in her sights even before she came to the UK, but one little comment from Harry such as 'You'll love her - she's been like a sister to me!' would have sealer K's fate. And the fact that Meghan then realised she was expected to kow tow to Katherine - well, that was it. Light the blue touch paper and scarper!
They've been a disastrous combo, especially for Harry. What he needed was a sensible, resilient, outgoing woman who (metaphorically) kicked him in the backside, told him to grow up and stop moaning about the highly privileged hand fate had dealt him.