I do agree that John Cena does get a lot of unjustified hate. The truth of it is that John Cena does get shit on a lot for common happenings in wrestling that can be applied to virtually everyone else. For instance, we all know about Cena's "5 Moves of Doom" shit that a lot of fans like to toss around. The fact is that every wrestler has a certain handful of signature moves that they manage to work into every match they have. Flair did it, the Andersons all did it, Stone Cold did it, The Rock did it, Kurt Angle does it, Jericho does it and I can keep going on and on with it. The simple fact of the matter is that some people just plain out hate John Cena just for the sake of hating hating him. They don't like him being as big of a star as he is nor of having the spot that he's got.
As far as The Undertaker goes, I don't really care if his WM matches are all the same. Why? Most of them have been at least solid and a handful of them are downright classics. It's true that the Taker's last 3 Mania matches have followed a certain formula but, so what? The WWE has built up the importance of the streak over the course of the past 5 or 6 years generally to such a successful degree that it's become part of Taker's identity. The past 3 years especially, these matches have told a fantastic story as well as been extremely competitive. A big part of what's made the last 3 matches so great in the minds of most people is the fact that they've been emotionally invested in them, especially the last 2 matches.
While I do think that a lot of the hate Cena gets isn't justified, he just hasn't earned the near universal respect that Taker has over the course of more than 20 years. The Undertaker has adapted his character in order to stay relevant with the changing times. Taker is every bit as over and respected now as he's ever been, maybe even more so, and most wrestlers that've been in the game for as long as he has simply can't make that claim. The last few years, it's definitely true that injuries have taken their toll and have really started to catch up with him. He's well known for working extended periods while hurt, pushing back taking time off to heal up or to have needed surgery, and he's started to pay the price the last few years.
If Taker just winds up working a single match a year, his WM match, now until the end of his career, people are going to rave about it if he's able to deliver storytelling on a level as has been done the past 3 years and rightly so. The fact that he can potentially deliver more in one match than most others can in an entire year of matches combined just shows how good he is in my view.