Well, to be honest, if it is good solid rain I think I would rather have my 8x10 tarp. Then I think what I would do is drape it over my gear and then set it up. There would be enough room under the tarp to dart inside, take off wet clothing, set up my sleeping bag and get inside.
I think I could drape my poncho over my gear and then set it up, but there would be no room to get under the shelter and then get in my bag. I think I could survive with the poncho, but survival is about the best I could hope for in solid rain.
For camping in Southern California, where I live, I think that the poncho is most excellent. It hardly ever rains here. And there are very few mosquitos here, too.
When I hiked the PCT last year it rained in the desert. All of a sudden I could hear rain falling on my tent. A situation like that I think the poncho could handle.