Slightly different target use cases. You'd use Rive for your UI, but not necessarily your player character and other things that you want animated "with" the gameplay. For that, you'd use Spine.
Yeah, although there are Rive examples using it as a game it's really targeted to vector-art your UI and allows you to create animations inside their software, which you then just play in whichever engine (such as Unity etc).
Rive does skeletal animation and a lot more.
It's a comparable tool, yes, and common in the industry, but I have not personally used it yet.
The Bay Area start up money bought nice marketing that makes it look popular, but it's not really.