In terms of work...
John Cena has nearly 20 years invested in this company, I am talking since 1999.
John Cena has also worked above and beyond full time from 2007 till about 2014, 7 years of busting his arse.
John Cena has been the face of the company, essentially, for the last 10 years.
Roman Reigns has just over 7 years invested in the company (2 spent in a stable, one in developmental) - so 5 years realistically.
Roman Reigns has yet to work the level John has for the WWE (talking in ring, out of ring, personnel, make a wish, trips to Iraq, ect)
Roman Reigns has yet to be pushed to be the legit face of the company.
Comparing them down strip:
Comparing Roman Reigns' WWE Career So Far to John Cena's Early Run
John Cena is one of the hardest workers in the company. Roman has no where near touched his level in those terms.
That aside, it is a popular opinion to hate on Roman which to me is not fair because he has improved. A lot of the hate is crowd sourced and found mainly online by people who simply want someone to hate and boo in the company. To compare Roman to John right now, that is hardly fair. Roman has a long way to go to be on John's level and it says a lot that WWE, Vince, HHH have made him "the guy" to take over that place which is why I don't understand the hate.
BUT to say John Cena sucks and Roman is indeed better all around, nah breh. John can put people over, has mic skill levels that are nearly untouchable even as a part timer, and has such an outreach to people that when people who has NEVER watched WWE see him, they think "WWE".
I am not saying Roman sucks, but let us be real here. John has made a legacy for himself and it is going to take more than 5 years to top that.