Depends on the purpose. I use the multiple outputs for resampling through external gear, for example. It works, but you have to be careful - you might literally fry your ears if you don’t know what you’re doing.
I’m gonna repeat that once more - be careful. The equipment can probably handle it, but your ears might not, if you don’t watch your levels.
But it works
For internal resampling, there’s a feature for that so no need to rewire the outputs to record an entire performance back into the blackbox, for example. It can do that on its own.
And it allows for internal rewiring between the outputs as well, though other than for resampling through outboard gear, I’m not sure why you’d want to do that if you don’t intend to send the audio through a mixer.
The onboard mastering section is actually quite efficient, so if you’re going for complete and final results, you’d do a lot worse than the blackbox alone. It’s a clean and bordering on harsh character on the four band eq included for mastering, but watch the frequencies and you’ll be fine. And the compressor, providing that you push it, has a subtle but very nice punch once you find its sweet spot.