The article link that Dave C provided does an excellent job of pointing out advantages and disadvantages. I have used tarps and poncho/tarps as my main shelter for a long time. If I expect a lot of rain, then it is a 8 X 10 tarp for me, with separate rain gear (often a poncho), and a bivy is not needed.
I have done quite a bit of rain weather the past few months using a zPacks Hexamid with a zPacks poncho/ground sheet. The shelter has done an excellent job even in slanted rain, when pitched at the right orientation.
Since the poncho is also the ground sheet, I can take it off when I am done setting up, and put it on in the morning when I start to take down. If I wear shorts with a 7" inseam, the bottom of the shorts get wet, which I don't like. So I have been wearing 5" shorts with it. The poncho is not as big and loose like most ponchos, so it is easier to hike in, plus I am used to using a poncho. I did some bushwhacking last year down a desert side canyon with lots of boulders, shrubs, cat claw, and cacti and the poncho was not a hindrance. Total weight of the shelter/poncho/groundsheet is under 7 ounces, sans pole and stakes. In groundsheet mode, the poncho is actually a bathtub floor with the optional zPacks clips. A bivy is not needed.