Other than the fact that we need another new token female companion every series.
Serena hasn't even had a single real battle since the saga began. Her pokemon has attacked TR a few times, but she's never actually battled a trainer. She is going the longest without capturing a second pokemon, over 40 episodes if the new Japanese opening is anything to go by/
Serena is probably the most utterly incompetent Pokemon trainer we've ever seen on the main cast. She treats her pokemon more as a pet than a Pokemon. It's like she's only there to be a bland girly love interest to Ash and nothing else. She seems to have no interest in battling at all, or doing anything Pokemon related.
I don't know why the female leads in the anime have been handled so poorly since the end of DP. I thought Misty was fine for the first token sidekick, May and Dawn were probably handled the best....but ever since Iris and Serena they've dropped the ball. Have the writers just given up on female characters after introducing so many?