Why is everyone saying that Primal Kyogre flat out sucks and is underwhelming compared to Primal Groudon and Mega Rayquaza? You do know that Kyogre is still the best of the 3, right? Nobody in their right mind would stay on Primal Groudon if it put the sun up last. Primal Kyogre and Mega Swampert is also a thing now. You do know that Precipipe blades does not get a boost in the sun unlike Origin Pulse, (rain) right?
Well, I'm not one of those people, however they've some points. Substantially, is true to assert that Kyogre is the one of the trio that gained the minor boost. Primal Groudon received a fantastic addition in fire typing, and thanks to its new improved ability it doesn't have to worry about x4 weakness to water (except for slower P-Kyogres and Mega Quaza), but not only that: it has now a great 150 base special attack, meaning that it can abuse its special movepool for devastating mixed sets, just think about the plethora of fire typing special moves boosted by its new STAB and ability, or other significant coverage moves such as solar beam. Also, when you're talking about the best of the 3, in my mind that one is definitely Mega Rayquaza! Delta stream is just amazing for it, the perfect ability for that typing; also the stat boosts are nearly as perfect (180 offenses, 100 defenses and 115 base speed, that's some SCARY flying dragon), and of course one of its greatest additions is its new signature move Dragon Ascent, a flying type STAB close combat, even better than brave bird and more spammable than outrage by far. Primal Kyogre isn't bad by any means, of course it'll still be one of the best uber pokémon out there, but it's quite undeniable that it gained much less than the others: the boost in physical attack is practically useless (maybe some mixed sets with EQ? Even giving that point, it isn't very exciting) and its signature move Origin Pulse is almost an hydropump with +5 PP and accuracy... A signature recovery move would've been much more useful IMO (and greatly OP as well, I'll give you that). The ability has improved a lot of course, but neutralizing resisted fire type moves for Kyogre isn't as useful as neutralizing x4 s.effective water moves for Groudon. So, my conclusion is that Kyogre was already an extremely good pokémon and GF decided to not boost it too much, while Groudon and Rayquaza where suffering much more competition in their environment, so they gave them great uniqueness to compensate.
And about Primal Kyogre + Mega Swampert... that would only works in multiple battles, of course not in singles as Primordial Sea will cease after switching out.