Last year I made the switch to a bivy & poncho tarp on "pure faith" that it would work out for me. (I did not have any friends who had a bivy & tarp for me to borrow or to see first hand).
So far, so good, but it does have its limitations.
To answer your questions:
I sleep on my back and I tend to sleep like a dead log. If you are and active sleeper, this could be an issue. Two things that I have for my Mountain Laurel Designs bivy is that I have a side zip and mine is extra wide. I highly recommend the side zip for ease of getting in and out of the bivy at night for restroom breaks. The wide cut gives me plenty of room to wiggle around or move a little bit so that I don't feel like I am in a coffin screaming to get out. The wide cut helps if you have a thick sleeping pad and/or a thick sleeping bag...like a winter bag. My bivy has a tie out loop at the top of my head area, which I clip to the underside of my poncho tarp. This has the effect of pulling the head area/bug netting up and away from my face, which also gives me more "mental breathing space". I am 5'6", so the extra space above my head is where I put my glasses, head lamp, extra clothing, which is nice....I have everything that I need for the night inside my bivy.
Site selection is one of the greatest things that I was looking forward to when I got my bivy. I can pretty much fit into the smallest places if I am not using my tarp. I have on a few occassion in the Sierras, just sleep out on the flat granite areas, which would be almost impossible to stake a tent down to. Setup on a night with no chance of rain is extremely fast and easy. Lay out the bivy, pop in your pad, and sleeping bag....zip up the bivy to keep the critters from getting in and you are done. (Assuming you are sheltered from the wind. Otherwise, I could stake down the bivy with the exterior loops. Have not had to do that yet.
Condensation: I have an eVent top on mine and condenstaion has been none to minimal. I have only had 1 experience where the condensation was soaking the inside of my bivy and the top of my bag. Really the result of dew point vs. a failure to vent. Nothing the sun could not dry off in the morning. That said, make sure that your head is positioned under the bug mesh so that your exhalation is venting out the mess. Early on, I sleep to "low" in my bivy and I noticed the condenation forming just under the bug mess...user error. Because of my relatively limited experience using a tarp/bivy and for my mental health, if I knew that I was going into a situation where I was expecting a lot of rain, I would probably take my Contrail do give me more room to move around, sit up, eat food, etc. That said, I don't see why I could not do the same in my bivy under a poncho tarp, but something to be said about more floor space when you are trapped for hours on end in a rain storm.
Riff Raft and Vermin: Has not been an issue. Thankfully, I sleep like a dead log, so if a bear or rodent came up to my bivy, hopped on top of my chest and started pounding on my head, I doubt that I would notice it. I just don't think that the animals want anything to do with my yellow and green freakish looking bivy. I look like a bright yellow twinky in the wild. I have spend two nights out when I unzipped my bivy and pulled the top back to sleep out in the open, which was fantastic to be under the stars and to be completely unrestricted. I did wake up with an bug or two in my bivy...small one, but I was fine and alive with all my digits.
Wind has not been an issue with my bivy and in really bad weather, you can and should lower the sides and back end of your tarp to provide additional protection.
See Ron's great photos below about how you can close up the end of your tarp to provide more wind and storm protection.
In my case, the eVent gives me a bomber bivy that is waterproof, which is probably overkill and heavier, but as this was my 1st and last bivy (because Ron's workmanship is just top notch) I opted for bomber upgrades for eVent top and silnylon 2.0 for the bottom.
That said, I have not used my bivy in any sort of extended rain for a good test...but there is no reason for me to believe that I would to adequately protected.
One thing that my friend, Jeremy, and I love about our MLD bivies (he has the Superlight and I have the Soul Side Zip) is that paired with a poncho tarp, we get the wind and rain protection but feel more connected with the environment. We can wake up in the morning and look around 360 degrees to see things. Lay on our bellies with the bivy opened up and simply enjoy the view, which is limited in a tent.
If you are thinking about a poncho tarp, they are a great way to cut weight, but the big draw back is if you had to setup your tarp while it is raining. You are going to get wet, though wearing a wind shirt would help.
Anyway, hope that this long rant helps you out, but a little over a year ago, I had the same questions and concerns that you have.
Good luck on your gear evolution.