The problem is that Aegislash is likely to come back for another bout in Ubers, given its ability to check Xerneas, and even univested Shadow Sneak could OHKO Lunala, even if the latter could deal substantial damage with Moongeist Beam (assuming it's a ghost move, damn you trollfreak), and will more than likely outspeed Aegislash under normal conditions, meaning that Lunala will get forced out by the shield.
A problem I see with Solgaleo is that its typing and ability means it directly competes with Mega Metagross. The only advantages I can come up with at this point are that it doesn't use up a mega slot and its signature attack could have a neat special effect, or at least be better than Meteor Mash. It's also got more bulk: someone at Smogon realized the Pokemon in the video all have 0 IVs and EVs based on that Gengar, and since we see Solgaleo's HP bar they calculated its base HP to be around 152, which surpasses even Giratina. If that's true, than our Metal Psychic Sun Lion could become one of the best walls in the game. Unless it's balanced with mediocre defense/special defense, 152 HP is patently absurd for a Steel Type, even if they're legendary.
True, although in that regard, mere speculation, but I would be surprised if aegislash saw less usage in the sun/moon meta due to solgaleo, for the same reasoning he would body lunaala. He's a viable counter, yet running both him and solgaleo would create an easily exploitable fire weakness. If the legends are anything metabreaking like oras/xy, solgeleo and lunaala will need dedicated counters, and while aegislash does this, primal groudon walls him more than he could wall lunaala, and aegislash isn't a mon people typically send to 1-1
Primal Groudon is nearly as high up there in uber usage as xernas. Aegislash doesn't want any of that, assuming it was a full uber battle, as long as you are running both P. Groudon and solgaleo, they would cover each other well, and be able to perform safely on a team
Yet the same applies to Lunaala and yet again primal groudon covering each other. Steel/Psychic physical tanky attacker solgaleo versus dark/ghost lunaala probably special attacker, lunaala
should beat it in most 1-1's, and primal groudons bodies most of what would stop lunaala (Aegislash, etc)
tl;dr Primal groudon bodies both lunaala's and solgaleo's most compatible supporting mon's, and yet, if it was solgaleo/p. groudon/x/x/x/x/ versus lunaala/p groudon/x/x/x/x, the 2nd team has an inherant advantage