The first thing I check on a halter dress is what's happening at the back of the neck, because that's where every single one either earns its keep or ruins your afternoon. A thin strap digging into bare skin for six hours is a different experience to a wider band or a tie you can adjust yourself. If I can't loosen it, I'm already working out how long I can realistically wear it before I need to go and sit somewhere with my arms down.
The honest limit of a halter is that it asks a lot of your shoulders and upper back, so it's not the dress for a day that's mostly about moving around, carrying things, reaching up into cupboards. It also does nothing for anyone who needs bra support with proper straps, because most halters either assume you'll go without or expect you to find a specific strapless or halter bra that actually fits, and that search is its own small nightmare. If the fabric is thin and clingy rather than something with a bit of weight or structure, it will show every line underneath, so this is not the dress to throw on and hope for the best. Fine for sitting, drinking, talking. Less fine for a long walk, dancing properly, or anything where your arms are above your head more than twice.
Getting the fit right means paying attention to two places, not one. The neck tie needs enough length to actually tie comfortably rather than strain, and the bodice needs to sit on the bust rather than under it, because a halter cut for a straight size scaled up will often put the fullest part of the dress a good few centimetres too high, so it pulls instead of skims. Go up a size if the only option is tight across the back, since a looser body with a well fitted neck does more for you than a tight body with a strap cutting in. Check the back too, a lot of halters leave a big expanse of skin there with nothing to hold the shape, so cheap fabric sags and looks tired within the hour.
What separates a good one from a sad one is usually the fabric and the finishing. A dress with a bit of viscose or a proper jersey blend will drape rather than crease into a map of your day, and a neck tie with a bit of width and a decent seam will hold its shape instead of twisting into a shoelace by lunchtime.
This suits anyone who likes their shoulders and doesn't mind doing the work of finding support underneath.