Feel free to disagree me, it's just my opinion. I don't care about mistakes, I know those use to happen inevitably. Luck made his fair share of mistakes too. When I judge a QB, I always judge by my eyes. Sometimes I see flaws which I think are correctable. By learning, or they come by experience, etc. But sometimes I see flaws which I believe aren't correctable. Or not correctable enough for an NFL starter level. (Or can't be compensated with something else.) For example a throwing motion is correctable on paper, but certain players just can't correct it. Tebow for example. Or Bortles. Siemian on the other hand has a beautiful throwing motion, good foot technic, he can read defenses, andhas the necessary arm strenght. But he has a problem with his decision making, every now and then he blocks and does something very stupid. He is like Hoyer. If you watch Hoyer, you can't see anything wrong, he looks like a solid NFL QB. Until his brain blocks and starts to make silly mistakes, and you can't explain why he commits them.
Regarding Brissett, the red flag for me is his slow mental processing. I mean veeeery slow. He is slow even when he knows what he's going to do, and all he needs to do is to execute. He's a bit late with even the most simple, quick passes. The Colts call a number of RPO plays for him. He is a good half - sometimes a full - second slower in his decision making than it would be necessary.
This has nothing to do with inexperience. If someone is quick in his mind, then, at least, he is quick when he knows what he's going to do. Brissett is not. I don't think this is correctable. Maybe a bit, but it will always be a problem for him, and I doubt he has enough other tools to compensate that. I wish I was wrong though, but I fear I'm not.
Btw, Russell Wilson has similar problem. He is a bit slow too when it comes to throwing (making the decision). That's why imo the Hawks constantly fails to implement some quick passing game, despite they've been trying it for years. But Wilson has plenty of other tools to compensate that. He has so many - his quickness, his extremely good pocket awareness, his touch, and one of the best and most accurate deep balls in the league, etc. - which not just compensates that, but makes him one of the better QB-s in the league. We can go through the NFL QB's one by one, and many - if not most - of them have flaws. Some of them has bad throwing motion, some of them are not that good in the pocket, some of them can't play from under center, etc. But all have something which they are very-very good at to compensate their flaws. I don't see such thing in Brissett. He's OK in many aspects of his game, but I don't see anything way above average.