This sounds like it was written by someone who has never used Eternatus. This mon checks Ho-Oh and (defensive) Dusk Mane, not the other way around. Life Orb sets can also 2HKO Primal Groudon and Arceus-Ground, usually the only 2 things on balance that can kill it, with Dyanmax Cannon. As for the "common Pokemon" that it loses to, I don't see any above A- other than Zacian-C, Primal Groudon, and Arceus-Ground.A to A-
Can't believe this was nominated up. Eternatus is really not good at the moment, especially with almost every team having Necrozma-DM, Ho-Oh and Primal Groudon on them. It rarely ever has a dent on the match and shouldn't be used very frequently. It loses to so many common Pokemon, and doesn't do much back. There are Pokemon in the lower ranks I'd rather fit onto a team than this honestly. Eternatus is not that good.
Really, Eternatus is just dummy good because it exerts so much pressure on balance; against this sample team, for instance, you can literally lead off with it and 6-0 without ever having to switch out. I completely stand by my A+ nomination.
I think this is still fairly effective in certain matchups. Being able to shut down support Dusk Mane (both Stealth Rock and Knock Off variants) is pretty substantial, imo, and without prior chip even a Dynamaxed Dusk Mane can't break through Mega Sableye. I don't think Magic Bounce is all that valuable anymore since Heavy-Duty Boots has lessened the need to keep away hazards this gen, but still, not D material.C to D
Stalls are starting to go without this due to its inability to effectively check common threats, Sableye-Mega just doesn't hold up.
0 Atk Necrozma-Dusk-Mane Max Steelspike vs. 248 HP / 252+ Def Sableye-Mega: 127-151 (41.9 - 49.8%) -- guaranteed 3HKO