There are a couple of companies, MLD, StoS, and Rab, that make quality poncho tarps that have been proven to work well as rain wear and shelter.
The poncho tarp is a super niche bit of gear and is only suitable for certain enviroments and pushing their use outside of this range requires skill, fitness, and the ability to embrace a lot of suck.
Prime poncho tarp situation:>Clear trails>Little undergrowth>Plenty of natural shelter (trees, large shrubs, etc)>Low risk of serious weather events
Unsuitable situation:>Bushwacking>Exposed campsites>High chance of strong wind or heavy rain>The user has little experience with small tarps
Cam Honan used a poncho tarp for most of his year long hike around the US and got on just fine, he wrote a great article on poncho tarps on his website, thehikinglife.
My personal advice to people insterested in poncho tarps is to first buy a small flat tarp and camp out in that a lot. The MLD poncho tarp is 9'x~4.5' in tarp mode. If you can stay dry and comfy with a tarp that size then you could consider a poncho tarp.