The first thing I do with any bra is put two fingers under the back band before it's even fastened on the loosest hook. If it's already tight there, it's not going to get better once you're wearing it all day.
A good bra will let you down if you pair it with the wrong top rather than any fault of its own, so it's worth thinking about what you're putting over it before you buy. A plunge with a very low cut front is wasted under anything with a high neckline, and a heavily lace cup will show every seam and edge through a fine jersey top, which looks fine in low light and less fine under office strip lighting. Push up styles with a lot of padding can look obvious under thin fabrics too, so if the top is sheer or clingy, go for a smoother finish and save the more structured ones for knits and tailoring where the shape gets absorbed into the fabric rather than announced by it. On colours, I've come round to nude and black doing almost all the real work, not because they're exciting but because they vanish under everything, including white, where a lot of people assume white bras are the safe choice and they're actually the ones that show through the most. A proper nude, close to your own skin tone rather than a vague beige, disappears under pale colours in a way white never will. Black is forgiving under dark and printed things and hides any slight discolouration from washing far longer than pale shades do.
Where I think bras genuinely let people down is fit promises on the label versus what actually happens on a body that isn't flat and symmetrical, which is all of us. A lace bra with no lining can dig in at the underwire if you've got any fullness at the sides, and a lot of push up styles are built for a specific proportion of cup to band, so if you're between sizes the padding sits oddly rather than lifting anything. Go up a back size and down a cup before you assume the whole style is wrong for you, that adjustment solves more problems than people expect. If the straps are already biting into your shoulders in the changing room, they will be worse by evening, not better, so don't buy on hope.
Best for anyone who tries it on, does the two finger check on the band, and actually moves their arms about before deciding, rather than just looking in the mirror standing dead still.