There are many ways to breach an embargo without being able to held accountable.
As I said, you can report on rumours and highlight certain things you know will be found to be true - so you can say with confidence, knowing you won't look foolish later.
You can also allow someone else, who wasn't privy to the embargo, to learn the details and then leak the information. A risky strategy of course, if you're found out, but the advantage now is once the info is out there you can perhaps report on what has been leaked.
Now I sign many NDAs and we're not allowed to comment on leaks and rumours once we've signed it, precisely for this reason, but it does happen.
As above, it is common to merely mention what is being reported elsewhere.
Embargoes are very useful for the press. I rely on them a lot for news and product announcements as it allows me to write in more depth that being sprung with the news at the same time everyone else gets it.
However, they are often abused and used ONLY to delay the release of news when it might be beneficial to do so. I might be told I can only report a new product at this time, and then talk about certain features at that time, and not publish a review of anything until another date (there are YouTube videos quite recently about dual embargoes).
This means a new thing could be announced today, available to buy immediately, and reviews can't be published until a week or two later. If the manufacturer is fearful of negative reviews because they already know of some issues, that delay is VERY useful as it allows a significant number of units to be sold in advance.
In politics, it's clearly a good way to bury bad news or at least reduce the value of it when it drops at some stupid time of day, maybe just before you know other big news is going to drop (or there's a big event or whatever).