You can honestly use any keyboard you want to do steno. But the Uni is specifically designed with steno in mind.
First, the layout of the Uni is designed so that it's very comfortable to do steno with, whereas some ortholinear keyboards might require your fingers to scrunch up a little bit. The classic thumb tucked behind the palm is a no-no.
Because the Uni comes with sloped keycaps, it's easier to chord multiple keys with one finger. You also want very light keys when pressing multiple at once, so it comes with Gateron clear switches which are only 35 grams each. If you want to go even lighter, to say 20 grams per switch, the plate conveniently has cutouts that allow you to open up the switch and swap springs without any desoldering.
To do steno on a computer, you need to install Plover, which is an open-source software. By default, the Uni can connect to Plover without interfering with your normal keyboard, which means you can use both at the same time.
All the things I just said can be emulated by an ortholinear keyboard if you take the keyboard and switch out the keycaps and springs and the firmware. But one thing that you can't do on a standard QMK keyboard is embedded steno, which is a plug-and-play version of steno that doesn't require Plover to run. You can read more about that here.
Lastly, there's a really cool [Plover] bird design on the back of the board.
Overall, the layout, keycaps, switches, switch plate, backplate, and firmware all combine to create a better experience for steno. Here's a link to the Uni product page if you're interested.