A bandeau bikini suits you if you have a torso you don't mind showing off without a strap arguing the toss over your shoulders, and if your chest sits somewhere between modest and a C cup without too much help needed. If you're larger busted, or you're after something you can actually move in without a hand hovering near your chest all day, I'd think twice. Length isn't my usual complaint here, oddly enough. This is one of the few things on a rail that fits me the same as everyone else, which almost disappoints me.
Worth checking before you buy: the depth of the band itself, because a shallow one on a tall ribcage sits right across the narrowest part and looks like a strip of fabric that's lost an argument. Look at whether there's any boning or rigid structure built in, because that's the difference between something that holds its shape on a sun lounger and something that folds in half the second you lean forward to pick up a drink. Check the grip too, a silicone strip inside the band or a bit of elastic at the top edge, otherwise you are hoiking it up every ten minutes and nobody enjoys that, least of all you. And look closely at how the straps attach, because the ones that come away and tie round the neck are a different bikini entirely to the ones that are properly bandeau and nothing else.
Where it falls apart is anywhere you intend to do something other than lie still and look nice. Swimming lengths, a volleyball game, chasing a child who's decided the sea is for running into fully clothed, any of that and a bandeau has a habit of travelling somewhere it wasn't invited. I wore one on a lido trip years ago and spent more time holding it in place than actually swimming, which rather defeats the point of being at a lido. It also doesn't play well under anything with its own neckline, a sundress with a scoop front, a kaftan that gapes, because you end up with band visible exactly where you didn't want a second horizontal line across your body. And if you're after support for anything beyond a B or small C, look elsewhere, this style simply isn't built for it, however good the marketing photography makes it look.
Where it earns its place is on an actual lounger, with an actual book, where the only movement required is turning a page and reaching for suncream. There, with a good grip strip, it's genuinely the most comfortable thing you can wear with no lines at all.