Misty, Brock, and Ash with a Pikachu at least has an amusement quality to their choice of names. May and Max were a case of them adding a female, who got involved in a canonical ship, and dragging along a 'younger-brother' character to complement this, as well as introducing Pokémon Contests in a way which perhaps got in the way of the focus on training, and in general while Misty couldn't half nag at least they occasionally tried to discuss battle and its own journey, rather than just having it come up in Contests. Although May could occasionally seem a bit out-of-place, which was appealing, this was generally not allowed to run for too long at the time, and she tended to just listen to people after a time. They didn't really form that great a series of companions, in some ways, and in any case it felt a bit weird to just add someone who was mostly superfluous, at that point you wonder if the anime wasn't too involved with tangential matters and feelings rather than the Pokémon.
Serena has a fairly poor plot-line, although this seems to be the standard for females in Pokémon games of late, but is decently reserved or uncomfortable at times, and there is still place to develop that. She isn't akin to Misty, at least, who often came across as just a part of the early Oak-Delia-Brock (less so)-Misty thing, and so on, which was just a bunch of people scolding Ash to press him into some form of obedience and reliance, and introducing another character who was mostly just there to tell Ash that they were bad at things, and to illustrate how Ash gets things wrong constantly - while seemingly by that point everyone else is portrayed as immaculate, other than Team Rocket who they might have been better off siding with at some point -, seems a bit unnecessary and doesn't really help things. Having them tag along with their gimmick and only form of dialogue at times being such things seems to pretty much limit Ash to losing things, in any case. Bonnie and Clemont still haven't lived up to their potential as the next Jessie and James, and this is disappointing, more or less. You might say Serena or Iris, in such a context, but obviously Serena's story is still going, so what occurs there might have to depend.
Incidentally, it's kind of weird how Ash loses the League some five times and somehow doesn't come across as that detached or resigned, by themselves, by that point. Perhaps they're not really focussing on the Pokémon battles or their relationship with that.