The first thing nobody tells you about white curtains is what the streetlight does to them after dark. You get a panel that looks crisp and clean in the shop, hang it up, and by nine at night it's glowing orange or sodium yellow from whatever is outside your window, because white fabric shows every bit of light that hits it, in both directions. Live with them a few months and you stop noticing, but that first week it can be a surprise if nobody warned you.
When you're checking them over, in the shop or the moment the delivery box lands, hold the fabric up to a window rather than a ceiling light. Thin white cotton or linen can be almost see through once there's daylight behind it, which is fine for a kitchen but not for a ground floor bedroom unless you're lining them or doubling up with a blind. Check the hem is actually straight and the panel width is generous enough to gather, because a skinny white curtain hanging flat against the wall looks more like a hospital cubicle than anything you'd want in a living room. Over time, plain white cotton holds up well to regular washing and keeps its shape, but anything with a slub or a loose linen weave will soften and relax the more you wash it, which some people love and some find looks tired. Spills and marks are the real test. Red wine, cooking oil near a kitchen window, a toddler's hands after dinner, white shows all of it, but it also bleaches and washes out better than almost any other color because you can actually use bleach or a strong stain treatment without worrying about the dye running. Iron them while slightly damp if you want that crisp look back, or just accept the lived in softness and stop fighting it.
Where white curtains let you down is in a room that gets hard use from cooking grease or a lot of dust, an open plan kitchen diner with no extractor fan will dull white fabric faster than you'd think, and in a rental with old single glazed windows they can pick up gray streaks from condensation along the bottom hem that take real effort to get out. They're also the wrong call if you've got a dark painted wall behind them and a lot of afternoon sun, because the contrast throws a strange glare that makes the room harder to sit in, not easier. Skip them too if you've already got a lot of white everywhere else in the room, white walls, white trim, white curtains, it flattens the whole space and you lose the sense of where one thing ends and the next begins. A patterned rug or a warm wood floor underneath does more to earn the curtains their keep than any white on white pairing ever will.