Meh... I don't think its game breaking. Sure its annoying but its like any other defensive ability in the game, you just have to plan for it. If you throw a DVA bomb, you just need to keep track of one more thing to make sure is on cool-down. You wouldn't DVA bomb if you knew Rein had full shield, so you shouldn't if you know he has immortality field
Whats annoying is potato teammates who don't shoot the damn thing when its deployed
It's not "game breaking," it's just stupid. Characters should never turn invincible, no matter what's going on. The fact that it's on a cool down and not an ult is even worse. It's a "shield" that doesn't require you to actually shield anything. You absolutely can throw a D.Va bomb if a Rein has a full shield because it can serve other purposes than to get kills, and I think it's an excellent example of why this ability is so bad. When a D.Va bomb gets thrown, the Rein needs to react to it by locating the bomb and then positioning himself properly to protect himself/team. So, a D.Va can throw a bomb behind a Rein or in a specific location to get him to turn or move. This new ability doesn't require anything other than pressing a button and the whole area is shielded. There doesn't seem to be an alternative strategy to defeating it, other than shooting at it, which requires it being thrown in the first place. It's just lazy design to me.
A big complaint of mine for games like Overwatch or League of Legends is that they constantly introduce new characters. It can be fun and exciting, but the more variables that are introduced, the more difficult it becomes to balance. A similar thing happened to StarCraft 2, which is one of my favorite games. With each expansion they added more and more units and it just reaches a point where it's impossible to balance correctly. It's not a "problem" that is unique to Overwatch, but I personally find it frustrating and also realize others enjoy that type of variety.