It seems rather nefarious, but there is Judicial oversight, meaning they need permission in each instance, which is at least one good thing.
But if a Russian hacker leaves Russia, or if a FBI hacker leaves the US, they can be prosecuted for those hacks. That's not what most people expect as the outcome of a "legal" action.
I think "government sanctioned" is the word we usually use for these cases, not "legal".
Of course they can't, FBI Hackers are likely liable to arrest and/or extradition if caught, while it's possible that the US might abrogate its extradition treaties in this case, suspects may find themselves arrested by 3rd countries the moment the leave US borders, or, depending on the country even tried in absentia
As for the FBI hacking outside their borders ...
Obviously, this is immediately problematic, no doubt.
But we live in a new, globalized world, and it includes not just social issues and business, but law enforcement.
A lot of pernicious crimes like sex-trafficking and child pornography etc. are very international in nature.
Ideally, we'd want authorities to work with one another, which is obviously the case in many ways ... but ... it's not always so easy. A lot of these regimes are totally corrupt, inefficient, have other priorities, or are politicized.
Remember that we are trying to achieve actual justice - and the rules are there to enable that - not the other way around.
I'm not exactly comfortable with what the FBI might be doing - that said, it depends a lot on what they are actually doing.
If the FBI hacks into the computers of some sex-traffickers in Burma, Ukraine, Belarus and is able to catch them ... then all the power to them. I think it's probably for the greater good, even if it does bend some rules.
Now if they start to go after political dissidents, and journalists, well, we have a problem :).
I'm not saying it's impossible, it would just be the first I'm hearing of it.
Just any international bank just about anywhere in the world with any US citizens as customers can tell you all about it.
Judicial review is likely now or soon to be a button in their hacking tools.
Where is this requirement located?