For years I thought the bracket was the whole story with wall shelves, buy the beefiest looking one and you're set. Then I watched a shelf stay perfectly level while the wall around it slowly crumbled into dust, because the real weak point was never the bracket, it was what I screwed it into.
These are wrong for the job in a couple of obvious situations. If you're renting somewhere with plaster over lath, or anywhere you can't locate a stud without an electrician's level of patience, floating shelves rated for real weight are going to fight you the whole way, and a freestanding bookcase will save your weekend. They're also the wrong call for anything you need to be load bearing and casual at once, like a shelf you want to lean cookbooks against sideways. Shelves hold weight best straight down. Lean things on them wrong and they'll bow, even the good ones, given a year or two. I've got a pine shelf in my garage that's held a permanent smile for three years now because I stacked paint cans on one end and left them there through two house moves.
Measuring up is where most of the damage gets done before the shelf's even on the wall. People eyeball the spot above a sofa and guess, and a shelf that's six inches too short or too long just looks like an accident, not a choice. Measure the piece of furniture underneath it, mark the midpoint, and let the shelf run no more than about three quarters of that width, sitting centered. Depth matters more than people think too, a shelf deep enough for actual books will stick out further than you expect over a narrow hallway, and you'll catch your shoulder on it within the first week, I guarantee it. Always find the studs before you drill, even for a shelf that's "just decorative," because decorative becomes load bearing the moment someone sets a plant on it, and drywall anchors alone will hold for a while and then let go all at once, usually when you're not home.
Wood shelves age the best of any version of this, they take a scuff and a sand and a fresh coat of oil without complaint, and real wood doesn't sag under steady weight the way cheaper composite boards eventually do, especially over longer spans. Wipe spills quickly, keep them out of direct sun if you care about even color, and check the brackets are still tight about once a year, because walls settle and screws work loose slowly, not all at once.
Get the wall and the width right, and a good shelf will quietly hold its shape for longer than most furniture in the room.