The simple answer is that it's devastating, especially on the ladder. Because most people tend to run gimmicks for the ladder, a boosted Xerneas is very difficult to stop without having appropriate checks, which most teams don't run. NatDex gave Xerneas another tool in Hidden Power, with which it can now muscle past said checks in Necrozma-Dusk Mane and Zacian even easily. With the increase in the number of Dragon-types as a whole, too, Xerneas also acts as a soft check for offensive Pokémon such as Zygarde and non-Band Rayquaza.
Xerneas has defined the tier since the beginning, especially by exploiting Dynamax to its full potential. It was one of the most anticipated Pokémon at NatDex AG's premiere, and it didn't disappoint at all. It's one of the best wallbreakers and late-game clickers in the metagame. Balance builds were forced to run no less than three fairy resists in Necrozma-Dusk Mane, Ho-Oh and Zacian-C. Primal Groudon was an emergency Pokémon but because most were offensive in the early NatDex metagame, it was a bit risky as a check. This is comparable to the extreme issues faced in teambuilding in Gen 6 to prep for Darkrai and in Gen 7 to prep for Mega Rayquaza. Xerneas typically forced Pokémon oftentimes seen on bulky balances, like Ultra Necrozma and Arceus-Fairy, to become almost non-existent.
The number of roles a typical Xerneas could play with its usual set on any team at a given time in a battle was baffling. It could be a wallbreaker, a late-game sweeper, a stallbreaker, a cleaner, a cleric and a wincon. It wasn't difficult for Xerneas to fulfil all of these roles at once because its usual Geomancy set could easily mould itself to suit its needs. Its move options were versatile; Moonblast + Thunder could be paired with HP Ground or Fire or even Substitute. Dynamax HP Fire can OHKO Necrozma-DM and Zacian-C after a Geomancy boost, and Dynamax Thunder deals with Ho-Oh, which are all of its regular checks.
The increase in the usage of Dark-types in Yveltal and Arceus-Dark augmented Xerneas' rise. Xerneas + Calyrex-Shadow soon became one of the best offensive cores in the metagame which can punish almost all builds with the ability to cover for each others' checks and counters easily. Xerneas fed on Calyrex's checks and Calyrex dealt with Xerneas'. Calyrex-Shadow could further Baton Pass Xerneas its Nasty Plot boosts to escape from Yveltal, which resulted in mind games versus Calyrex; the odds were that the Xerneas user would be better off after the Baton Pass. The sheer prowess of Xerneas helped it rise in NatDex AG as one of the best offensive threats, and its offensive capability remains unparalleled.
