Too bad things went south for him in 2015 were he missed half the season with major injuries (3) that all required surgery. He wasn't the same after those injuries.
His injuries were a reason he was not likely a viable starter anymore. He also expected too much money, and to be a starter which hurt his chances. Also, when healthy, he ran too early, and ran around too much, which is very hard for an OC to gameplan for. But he was absolutely without question well capable of being a top tier backup QB once he fully recovered, no doubt. And as good as many other starters in the league with his dual-threat ability.
But that's not why he was out of the league, and we all know that. He refused to stand for the national anthem, never told anyone about it, just did it, and when someone noticed, it created a sh
itstorm. Granted, he didn't make it better at all when wearing a shirt with Fidel Castro (and Malcom-X) and talking about Cuba. Then wearing a pair of socks with pigs on it, which was taken as a blunt protest against police. The good ol' boys of the NFL owners group didn't like any of that, and that was the end of his career.
However, in the many months, now years, he's been asked about all this, his responses have always been fairly articulate, if a little pointed. But I followed him for a bit and he was actually rarely controversial, if at all. Mostly he's just a civil rights, human rights activist, talking about positive things people have done, groups and people who make a difference (recently talking about Cuba, basically just pointing out their free education, and universal healthcare). You had to really dig to find anything he said or posted that even gave you pause. Shame only
one single team even gave him a workout (Raiders), and I don't think he ever really was going to get signed by them anyway. They had Derek Carr, and had recently traded for Jarett Stidham to be their backup, and had Nick Mullens and Chase Garbers in camp too.