It's funny, guys who have zero chance of being good enough for WWE tend to be the ones that take shots, while guys who leave and still have a shot at a career - WWE or otherwise - speak nicely about it all. It's almost as if the ones "speaking out" have nothing better to do, since they have nothing else going for them.
Matt striker had a great interview where he said nothing but good things about cena and about how all the backstage rumours are heavily exaggerated
No, the ones who take shots are the ones with nothing to lose, who looked at WWE as the past and nothing else. Look at Punk, Tyler Rex, Chavo Guerrero, Michael Tarvar, all who have officially moved on from WWE and had legitimate reasons for disliking Cena.
People on the current roster do take shots at Cena, though not a lot; look at Ziggler and Cesaro (both have stated they are a little tired of seeing Cena vs Orton 10000th times) and also Ziggler stated that Cena's character is stale. Ziggler and Cesaro both got a depush as a result.