Echo chamber usually implies that the only voices being heard are those in agreement. Anyone who disagrees gets curated out one way or another. This leads to people believing they are right and people who disagree are wrong. It validates their beliefs and reinforces them. It allows them to other those who disagree.

It’s incredibly unhealthy and leads to radicalization. Just look at all the echo chambers that have led people to commit mass atrocities. Sure, there’s likely a preexisting likelihood of radicalization, but having everyone around you telling you how evil somebody else is can’t be a good thing. There’s so much disinformation that doesn’t have an opportunity to get debunked. There’s hatred of various groups that is being reinforced and propagated.

Incels killing women and couples because they deserve sex with anyone they desire because they were.. born? I honestly don’t know the reasoning there. Trans folk being targeted because they’re… bad for children somehow? Or something, I don’t get that one either. People dying of covid after taking their horse dewormer. People driving to other states to shoot a bunch of people to “protect”… something? The problem becomes that when all you hear is one thing, it becomes fact and no longer up for debate. If people were exposed to more of those they hate, there would likely be less hate in the world.

But hey, who hasn’t been there? It’s easier to hate and sit in your bubble knowing how right you are instead of talking to people and trying to change their minds. And if you don’t let their voices be heard, how can you help them grow? How can you grow as a person if you’re only ever exposed to those who agree with you?

its only group think in that a lot of the group independently think the same thing.

Groupthink is a psychological phenomenon that occurs within a group of people in which the desire for harmony or conformity in the group results in an irrational or dysfunctional decision-making outcome. Cohesiveness, or the desire for cohesiveness, in a group may produce a tendency among its members to agree at all costs. This causes the group to minimize conflict and reach a consensus decision without critical evaluation.

Emphasis mine

https://en.m.wikipedia.org/wiki/Groupthink