A bag charm should be one good decision, not six noisy ones, and the second you hang more than one on a bag at a time you've usually gone past charming into cluttered. I like a single piece doing a clear job, a flash of colour or a bit of weight that makes a plain bag look chosen rather than just carried. Pile on three or four at once, especially if they're all shiny or all the same scale, and they stop reading as personality and start reading as a key ring that lost an argument with a jewellery box.
The gap between a good one and a bad one shows up fast, usually within a fortnight. A cheap charm is light in a way that feels wrong the moment you pick it up, the plating goes patchy at the clip first because that's the part that takes all the friction, and any fabric or tassel element frays at the ends rather than softening. A well made one has a bit of heft to it, a clasp that clicks shut properly instead of needing to be forced, and finishes, enamel, leather, metal, that are solid rather than printed or sprayed on. Check the ring that attaches it to the clip too, the cheap ones are soldered thin and bend out of shape within weeks, and once that ring distorts the whole charm swings at an angle and looks sad rather than jaunty. It should clash with nothing you're wearing because it isn't trying to coordinate with your outfit, it's decorating the bag, but it will clash with the bag itself if the hardware tone is wrong, gold charm on silver fittings looks like an afterthought rather than a choice. Match the metal, or commit properly to mixing them on purpose, there's no comfortable middle ground.
Where it falls short is obvious if you're hard on your bags. If you're someone who throws yours on the floor of the car, in and out of bags for the school run, or onto a bus seat, a charm with any delicate tassel or beading will not survive that treatment and you'll be picking up loose beads off the pavement within a month. It's not the choice for a bag you use for work and nothing else either, it tends to look like decoration on something that's meant to look efficient. Treat it gently, keep it away from water and perfume which both dull plating over time, and take it off before the bag goes anywhere rough. Looked after, a good one will outlast several bags and simply move house each time, which is more than most accessories manage.