Something about the super mechanics that always annoyed me. When Mega Evolutions were introduced in XY, Masuda explicitly said that the reason that Mega Stones existed was to make sure Mega Pokemon were limited by losing their item slot. A similar story with Z-Moves: It was one time only thing, and once the move was exposed, you effectively had a useless item slot. Not to mention Z-Moves and four Attacks was worse than running a LO, Choice Item, or heck even expert belt with four attacks and a wide movepool. Plus the fact that Z-Moves weren’t strictly used Offensively: Z-Trick Room and Haze were great ways to play around countermeasures like Taunt and Imprison.
Which I always found it bizzare from a design and balance perspective that Dynamax and Teraastalization never allowed the item slot to be sacrificed. Especially in the former's case, since one reason why Dynamax was seen as so uncompetitive was not just the unpredictability, but the absurd combos like Weakness Policy, Life Orb, and Scope Lens for Togekiss's case. If there was an item slot, not only would those unpredictablility be significantly reduced but it would ensure those absurd combos would exist.
I'm guessing GF is more keen on trying more unpredictable mechanics is because they know that VGC is Bo3 and unpredictable mechanics are less unpredictable when you have matches to fall back on as well as the new open team sheet ruleset ruining surprise strategies. But still that doesn't take in account Battlespot Singles, the official cartridge ladder, since that meta is also hosted by GF and has a huge portion of the Japanese playerbase, and Dynamax was significantly harder to handle in 3v3 compared to VGC from my understanding .