I think it's obvious that Cena has decimated Reigns in their promo exchanges. Reigns has improved on the mic, he's not even remotely close to being on Cena's level but he has gotten better. One reason why Cena scorched Reigns so definitively is because Cena is still being protected; if allowed, there are places that Roman could go that, from a wrestling promo standard, could take the wind out of Cena's sails.
Cena does indeed spin, or rather he and the writers both spin, the various criticisms made about him by numerous internet fans over the last decade plus in ways in which render those criticisms as ineffectual. We've heard all the various "you can't wrestle" nonsense or the "Cena buries" guys stuff from others before Reigns and it doesn't really work because all Cena has to do is remind people of his work ethic. Like him or not, NOBODY has had John Cena's work ethic in WWE, NOBODY has gone all out in order to hype and expand the company as Cena has, including becoming fluent in foreign languages. The most hardcore Cena hater can't argue with that, they can try but they'll instantly look foolish the moment they do. As for him not having great matches, that may very well be the single dumbest thing to come out of Roman Reigns' mouth since the infamous Sufferin' Succotash promo a few years back.
If WWE wanted to level the playing field a bit when it comes to these promos, I think the key would be to have Reigns ultimately do what Cena does against him: mix in some of the various fan criticisms about him with real life goings on. For instance, Cena mentioned Roman's failed drug test from last year. That won't happen, however, as Vince will do anything to protect the image he's helped build for John Cena, so that leaves most opponents pretty hamstrung as all they're left with are the old school heel tactics and internet fan complaints. If Vince did allow it to go down, I could see Reigns saying something along these lines:
Roman Reigns - "You know why I've called you a fake ass bitch John? It's because that's exactly what you are. You come out here week after week and deliver these sermons from the mount, the gospel according to Cena, in which you run down your opponents and your detractors with your A+ spin tactics. The central part in all your sermons is your mantra of Hustle, Loyalty and Respect and I wanna talk about that for a bit. Y'all know what a hustler is? A hustler is someone who works hard, does their absolute best. That's only one definition though, see there's another one where a hustler is a con man, a scam artist, someone who does and says whatever they have to do in order to get what they want. That's what you are John, you're a con man, you're a scamming, lowlife hypocrite who calls people out for the sorta conniving, backstabbing crap that you pull. As for Loyalty and Respect, well they're pretty self explanatory right? You don't know a damn thing about Loyalty and Respect John. Hell folks, y'all don't have to take my word for it, all you gotta do is hit up Cena's ex wife and you'll find out just how loyal and respectful to her he was. You come out here John each weak and you paint yourself like you're some great American hero, like you're Superman or something. But you ain't Superman John, I'm pretty sure Superman never screwed around with Wonder Woman behind Lois Lane's back and there've been a LOT of different gals playin' the Wonder Woman to your Superman over the years. You see John, it's not so much that I don't like you as a wrestler, I don't like you because you're a lying, fake ass bitch that's spent 15 years selling lies to people. Hustle, Loyalty and Respect to you is nothing more than a slogan to put on ugly ass t-shirts that you con little boys and girls to get their parents to buy for them. You always like to tell your detractors "You Can't See Me" but that's the thing John, I do see you. I see who you are, what you're like and what you do when the cameras stop rolling and all the little boys and girls you've conned into believing in you are home snug as a bug in a rug in their beds. I know who the real John Cena is, I know what he's really all about and it ain't this corporate, ass kissing mascot that you show everyone who isn't back in that locker room. John Cena is all about John Cena and that's fine brother; hell, I'd shake your hand and call you a stand up guy if you actually had the guts to come right out and say that. Now me, I'm not perfect and never said I was. I don't pretend to be somebody that I'm not, I don't try to shove a lie down people's throats the way you have for the last 15 years and I never will. After No Mercy, you won't be just the lying, scheming, fake ass bitch that you already are; when No Mercy's in the history books, you'll also be Roman Reigns' bitch. Believe that."
