Here is where we get into some options. A large tarp like a SpinnTwinn (I have one) does not need a bivy or other supplemental protection from the elements because it is so big. With something like a SpinnTwinn or my BPL Nano Tarp, I take a rain jacket (Marmot Essence). So with the Nano and Essence I am looking at around 12 oz for shelter and rain gear. Probably the most convenient set up.
But the Hexamid and Poncho/Groundsheet is under 7 oz, about 1/2 the weight.
I get as good as coverage with the Hexamid, unless I have sideways rain coming in the front of the shelter, which has not happened with good orientation.
Now the problem with a Poncho/Tarp is your shelter is your rain gear... so you will get wet during set up and take down in bad weather. Takes practice to minimize this.
But with the poncho/groundsheet you set up your shelter then while under it, you take off the poncho and make it the ground sheet. During take down, you pack everything except the shelter and groundsheet. Turn the ground sheet into a poncho then take down the shelter and put it in the pack and start hiking.
What I like with the Hexamid and Poncho/groundsheet is the bathtub factor. Joe really tricked out the design here.
I am 5'11" and with a small pack (zPack Zero small) the poncho bottom is above my knees. With longish shorts the bottom 2 inches of my shorts get soaked in rain, not a big deal in warmer rain, but not super comfortable in cold windy rain. 5" inseam shorts don't get wet. Now, I don't know how it is going to fit over my new McHale Bump and I have not used the poncho with my McHale LBP. But I have a cuben kilt that I have not used yet.
I do have a concern using a light cuben fabric as a groundsheet. Worse ground for ground sheets is desert where you have sand and small pebbles/rocks. But so far, so good. If there is no chance of rain, then I just use a 1/8" foam pad as a groundsheet and keep the poncho/groundsheet in my pack. I never set up the shelter unless bad weather is expected.
Seems everything is give and take.