I can answer why people don't like Reigns.
After the Shield ended, Reigns was popular. His match with Orton at SS was where the problems started. People noticed the match was sort of boring and got a sense of there is something not quite right here. Then came the injury. He is still fine at this point. Then came the promos. This is where the booing started. His promos were awful and WWE made no attempt to cover this up. His one at TLC that year was particularly bad. The momentum against him was picking up steam at this point. Him not being Daniel Bryan and winning the Rumble was where the hate went full speed ahead. However, the hate was coming anyways. A lot of people realized Roman was not ready for his spot. The Rumble only quickened his hate and was not the only cause of it.
WWE proceeded to give Reigns even more awful dialogue. That pretty much cemented him getting booed. They did not hide Roman's weaknesses. His flaws were being exposed constantly. That is why he continues to be booed. He got cheered when he was a monster. They so far have only booked him like that a few times. Roman has been horribly booked and that is why he is booed.
For the most part people claimed they disliked him because he was being forced down our throats a la John Cena.
I saw plenty of people say they wouldn't hate him if it weren't for that.
It's more about raging against the WWE machine than it is about Roman. He just happened to be the handpicked guy. There's no way he was going to survive that.