The shoulder strap should be well padded and non-slip.

They can be picked up and put down very quickly. That makes it easy to walk a stretch of water and set the bag down without any fuss whenever you need to.

They place one-sided strain on the body over long distances and tend to slide off your shoulder while you fish. They also fill up and get heavy quickly, which puts uncomfortable pressure on the shoulder. All in all, they are not particularly ergonomic.

Some come close to being waterproof. They are made from water-resistant material, but most do not have fully sealed zips, so a small amount of water can get in. If you need complete waterproofing, look at waterproof dry sacks or rucksacks instead.

Yes - so-called sling packs take the best of both options. They sit on one shoulder but rest against your back, so they barely get in the way while you fish.