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If anything the XYZ trio are the most balanced ones, Xerneas and Yveltal were the only Kalos mascots back in gen 6, barely anyone cared about Zygarde. Xerneas and Yveltal were also overwhelmingly strong and everyone had their eyes on them, even when ORAS came up with the Primals these two were still damn useful.
Eventually the anime reached the XY&Z arc and people wanted the Zygarde game already but instead we got SM with Zygarde's forms there, these made even people that don't watch the anime pay attention to Zygarde, wonder what happened to its game and all, and the mon itself was a menace with that bulk and Thousand Arrows, all that gave it its own loyal fans wanting more Z, but Xerneas and Yveltal were still strong and not even remotely forgotten. By the time gen 8 happened the focus on them cooled down but they were still strong and more people feared Zygarde because beating it in Dynamax Adventures was fucking hell thanks to the HP boost, but even without that Xerneas and Yveltal were still strong, they didn't have gimmick forms or mechanics or anything like that but they are just so god damn good competitively you could never completely forget about them, and now despite not being programmed in SV, unofficial metas still keep Xerneas and Yveltal as the absolutely insane menaces they are and the more casual fan has its eyes on Zygarde for its new form and refreshing game.
They are all great, relevant and amazing, I don't know how they did it, but these three are a perfect legendary trio.