Watched a "Hoenn starters&legendaries vs Sinnoh starters&legendaries" video earlier and honestly... the Sinnoh trio are really underpowered compared to most other legendary trios aren't they? In terms of abilities, specifically.
- the weather trio get to bring weather with them, making them far more powerful (kind of in Rayquaza's case - Mega Rayquaza corrected this for a time with strong winds)
- the tao trio get their own versions of Mold Breaker
- Xerneas and Yveltal get an ability which powers up their signature moves
- Solgaleo and Lunala get their own versions of Clear Body and Multiscale; this one's kind of skewed as Shadow Shield is far more generally useful but they're both undeniably good abilities to have
- Zacian and Zamazenta get an automatic stat boost whenever they're switched in (this was nerfed in Gen XI, but is still excellent)
- Koraidon and Miraidon get an automatic weather/terrain setup whenever they're switched in
While the creation trio just get... Pressure. Quite underwhelming really, and doesn't do anything to make them more powerful or make their signature moves more effective.
Relatedly, Telepathy makes sense as an ability for Origin Palkia (even if it is useless in singles) but it doesn't really suit Origin Dialga; I've seen all kinds of theorycrafting posts for more suitable time-themed abilities. Even Levitate would have been more beneficial.
I wonder if they wanted to focus more on signature moves rather than abilities. Kyogre, Groudon, and Rayquaza didn’t get their own signature moves until ORAS, but they did always have signature abilities. Meanwhile, the only Gen 4 Legendary that got a signature ability was Regigigas (and that’s obviously not intended to be to its benefit), whereas the mascot dragon trio, Heatran, Regigigas, and Cresselia all got signature moves. Feels to me like that was the new toy they were playing around with at the time. Then they started experimenting with combinations of the two in later generations.