It feels rather incomplete without the remaining legendaries, too.
Landorus with Earth Power would charge ground attacks slightly faster and be more threatening to steel types - even though so many pokemon already have ground coverage.
Zekrom would finally make a hard counter against Kyogre. Kyogre truly is a pain in the ass if you’re nowhere prepared for it. Plus it can munch through dragons…
Reshiram would make a huge impact as a closer, and threatening the very common Melmetal and Metagross
Thats just to name a few. But yeah it feels like its not really possible to cover the meta. Just have to roll the dice.
As for myself I’ve learnt which pokemons perform as others say, and which pokemons absolutely don’t through just trying them out (like Giratina-A is hyped in UL but I find it not a threat at all without the boost from AP. And same for Dialga here, so many options to OHKO this thing).