Not to mention Braixen suddenly having a decent skill and battle style out of nowhere in Serena's last episode and being able to hold up against Ash's Pikachu. Oh, but again, they call it off before Serena's team start losing too bad and while she still has the upper hand, heaven forbid she look overpowered by the longest running Pokemon in the series.
Yeah the end of Sylveon's character arc, considering how much they put into Eevee, was very disappointing. I hate when they take character development to the point they abolish all flaws in the character. That's not development, that's dumbing down, and likely the reason we barely even saw Sylveon afterwards, since she'd ironically become the blandest of the team. You can make a character seem fully developed without taking out all their flaws and foibles and just about anything fun about them, but then again look at what the same series did to Ash.