Pros:

- lightweight - [my bivvy bag][1] weighs 1kg, is Gore-Tex lined and cost about 80EUR some time ago
- water resistant - I've slept in perhaps 20mm rain staying dry
- you see the stars
- setting it up takes 10 seconds; the fastest I've been able to pitch a tent is 5-10 minutes and that's unpleasant in heavy rain
- because of the low profile is infinitely stealthier than even a 1-person tent; depending on the situations you get yourself into that can be an advantage or a disadvantage
- the low profile means high winds just pass above you
- doesn't require a rectangular flat spot to erect; I've slept on a steep incline jamming my stomach against a tree - good luck doing that with a tent

Cons:

- I've seen a frustratingly low quality bags
- your luggage is out in the rain
- no personal space to change clothes or read a book in the wind

Subjective conclusion:  
Seeing the lengths of both lists it should come as no surprise that since trying it, I use my tent **only** for beachside vacations or frontcountry BBQ. Climate: southern continental Europe.

  [1]: https://m.media-amazon.com/images/I/61yOa-jx4+L._AC_SL1155_.jpg