Okay i'll start this off with saying, Nagato gets overrated like shit on this site. Crud that he could beat the first Hokage, he could blablabla. So, don't hate on the cunt for overrating someone else against Nagato. Fanbois have the rights to be pissed, especially Sasuke fanbois.
Now, point; What can Sasuke realistically do against Pein? SM mode Naruto got crushed down by a Pein without the intention to kill. Literally all the advantage he had went poof the moment Deva came off cooldown. Imagine Deva + All the paths that were 'unfairly' lost. This, coming from a Naruto who had an advantage on Pein, in matter of information and on a skill level. Granted, he wasn't on a higher skill level than current Sasuke, but he did have advantages in terms of abilities. What i mean is, he had Sage Mode, which essentially gave him technically infinite chakra for five minutes, along with boosted taijutsu. Which gave him an advantage against Preta and Asura path plus, even if human path was there, she'd probably just been snuffed out instantly. Second of all, he had his summonings which did hold down Animal path's summonings long enough. That only leaves us with Deva and Naraka. Naraka being defenseless once these guys are taken out and Deva's on cooldown.
On the other hand, Sasuke has no individual counters to all this. Yea sure, he has a Susano'o. But, think about this, Susano'o, as we've seen abundantly clearly. Restricts your mobility like hell. So, if Animal path summons a dog out, is Sasuke going to sit there and shoot arrows at it infinitely while it trys to maul down on the Susano'o and keeps replicating?
Sure, he has Amaterasu. Sure, let's assume it can't be sucked in by Preta, can't Deva push it off easily? Okay, let's say we give Sasuke the same advantage Naruto had, that Deva is shut down for about ten minutes. He hits one of the paths with Amaterasu. It's gonna take amaterasu a while to do some decent damage. Meanwhile, the Peins can simply.. I don't know, take off their Akatsuki robe and boom, all that chaakra down the drain? Then, Naraka path can resummon any of the dead Peins and given how long Amaterasu takes to do decent damage. He can just you know, get those guys back. Then Asura paths nuking ability, against this. Without Susano'o, Sasuke's a dead man. Think of those bullet speeds and Sasuke's not too impossibly amazing movement speed. Naruto only dodge this bullet cause he took out Asura path first.
So yea, Sasuke's not getting underrated here, Nagato is just haxxed massively, also.. EMS, not done much for Sasuke or anyone yet.