So far, the only shortcut that looks to be useful at all, and not a duplication of existing Siri functionality (for me speaking American English), is the translate text function, which is itself very limited by Safari’s horrible text selection behavior. Also, I have the Google translate app, so...
I’m probably going to delete the app and forget about this “new iOS feature”.
If you’re in Apple’s tested speech range, Siri is very useful for extremely basic commands and statements. Go outside the most average vocabulary, though, and it looks as stupid as it actually is (entirely stupid; there’s no such thing as AI, just human-made, algorithms, scripts, and some occasional modifications of them via user input). Speech to text is abysmal for anything even slightly off the most average, or lowest common denominator.
Accent? Actually I think others already asked you this.
I was born and raised in Nebraska. Supposedly we have one of the most neutral accents in the US, and many news broadcasters were either born or trained in this region, from Kansas/Missouri to the south to the Dakotas and Minnesota in the north.