I've owned my Ursa Mini 4.6K for about six months at the time of writing, and I must say, I absolutely love it. I've been able to use it on a huge variety of projects, from a bunch of short films, to corporate work, music videos. I've rented it out to other CInematographers who've given very high praise to the images it produces. So I can easily say it's the best camera I've owned.
The first thing I'd like to talk about is the dynamic range. Blackmagic claim 15 stops, which is a fantastic amount. Other people have scientifically tested this to confirm the real amount you get, and I'm not going to go into that, but I can say it's definitely massively better than the old 4K sensor. Although they share a similar name, the two cameras are incomparable. Think of the Ursa Mini 4.6K as an Alexa and the 4K as a RED - that's the best comparison I can make. The 4K sensor was terrible in low light situations, and had quite a harsh end to the curve in the highlights part of the dynamic range. The 4.6K sensor is pretty good in low light. Of course it doesn't compare to a Sony A7Sii, but it was never meant to. The human eye's sensitivity is around 800ISO equivalent, so for normal situations, the pretty clean 1600ISO of the Ursa Mini is plenty.
So in real world terms, you can shoot streetlight stuff with no problem in cities with fast enough glass, but if you're shooting in a small town with quite weak streetlights, then you have to plan for that and bring lights - but honestly, you should do anyway. Even if you have a camera capable of shooting with nearly no lighting, your footage won't look cinematic if you're not spending time making it look good.