My first pair came in a set of two from a supermarket, the kind with a cartoon fox print, and they shrank in the wash until the sleeves sat halfway up my forearms like a jumper I'd stolen from a smaller sister. What they taught me was that cheap cotton and a hot wash are not friends, and that I'd rather pay a bit more and have them last three winters than buy the same sad set twice a year forever.
The honest limits are worth saying plainly. Pyjamas are not loungewear you can answer the door in and feel put together, and they're not something that survives being worn as daywear no matter how good the top half looks on its own, because the trousers always give it away, that slightly too soft drape that says bed, not out. The elastic waistbands on the cheaper ones also give up fast, going slack within a few washes so you're hoiking them up by week three, and anything with a very fine or delicate fabric tends to pill under a duvet faster than you'd think, going fuzzy at the hip and elbow where you actually lie on it. If you run warm at night, thick brushed cotton is the wrong choice full stop, you'll wake up at two in the morning fighting your own top off.
As for what they clash with, don't bother pairing a bold printed pyjama top with a patterned dressing gown over it, I did this once with a floral top under a paisley robe and looked like a settee. Heavy, structured slippers look wrong with soft pyjama bottoms too, the stiffness fights the drape and you end up looking like you're off to the shops rather than settling in. And resist layering a pyjama top under daywear to save time in the morning, the collar always pokes out wrong and the fabric moves differently to everything else you've got on.
What they do sit well with is simple. A plain robe in a single colour, something in a soft grey or a deep berry, lets a patterned pyjama set do all the talking without competing. Flat slip on slippers, the kind with a bit of give in them, move the same way the fabric does. And a plain fitted vest under a pyjama top on a cold night does more for warmth than another blanket, because it's the gaps at the neck and cuffs where the heat actually escapes.
The one mistake to avoid is buying by the print alone and ignoring the rise of the trousers, because a waistband that sits too low will slide down all night and you'll spend the whole week hauling it back up instead of sleeping.