"Dry", and especially "safe", are two very subjective terms that mean different things to different people.

Noticeably absent from your requirements is the word "comfortable". Are you looking for something that will just keep you alive? Beware of sacrificing comfort and safety in the pursuit of shaving ounces.

Most of the uberlite bivies attain their low weights partially because they are lower volume and/or smaller in girth. If you're a big person their girth may constrict you. They may also compress the loft in your sleeping bag thus lowering its insulation value.

Check out Ti Goat, a site sponsor. They make a very light bivy.