If I had a phone set up like that, and, say, ICE or TSA took it, what would they be able to get from it?

Depends on what state it’s in. If it’s in lockdown mode, nothing. GOS blocks all access to data via the USB. If it’s unlocked, everything that’s not locked by further authentication.

but what’s to keep them from just beating it out of you

Nothing. That doesn’t mean you should willingly consent to it.