A bivy sack is the lightest effective shelter that is a great protection against bugs, rain splash and extra warmth if weather turns nasty while tarp-poncho is the ultimate in lightweight versatility (a shelter, rain gear, and pack cover combined).
I’ve been using this simple shelter system that weathered countless trips on various parts of the Philippines, from a rainy cold night at the heart of the Cordillera mountain range to a windy night on a summit, and to a starry night on a pristine beach for 4 years now. I have no complaints or whatsoever. I stayed warm and dry inside my bivy sack on a rainy windy cold night under my properly rigged tarp poncho.
This is a great shelter system for bikepacking, fastpacking, adventure racing and for committed minimalists.