Off the top of my head I recollect they were intercepting hardware like switches and compromising it, inserting spying equipment in Google data centers forceing them to now to end to end encryption and many other shady things.
Not to mention the collection of zero days that almost certainly will get discovered by other nation states and bad actors and risk our security. They used parallel construction with other law enforcement orgs as they leaked run of the mill criminal behavior (not national security related) so as to not disclose the real way they got the info.
There are many more examples of overreaches he brought to light but I don't recall all of it. The core concern I have is all this apparatus are tools of fascism and control of any dissenting voices including the opposing political party. Where we are politically today I think would be hard to fathom back when Snowden leaked all this.