The honest limits
Thin ripstop nylon keeps the folded size tiny, and it also means no padding. There is no laptop protection worth the name, and shoulder straps without foam start to cut after a few miles under load.
What to look for
Check the zipper quality first, because it is the part that fails on cheap packable bags. Then look at whether the bag folds into an integrated pocket or a separate stuff sack, since a separate sack gets lost within two trips.
Weight versus durability
Under 200 grams means very thin fabric. Around 300 to 400 grams buys a denser weave that survives being dragged across a hotel floor. That difference is invisible in your luggage and very visible after a year.
