Most scented candles are a waste of wax, and I say that as someone who has bought far too many of them. The ones that smell incredible in the shop go dead quiet after an hour burning at home, and that is almost always down to a thin wick and a cheap wax blend, not a bad scent choice on your part.
Candles and diffusers earn their keep most in the rooms you sit still in for a while, bedrooms, living rooms, bathrooms, the kind of small apartment hallway that needs something to say hello when you walk in. Diffusers do steadier, quieter work and suit rooms you cannot always watch, a nursery, a rental where a naked flame feels like a bad idea, a kitchen where you would rather not leave a candle burning while you duck out. Candles come into their own through autumn and winter when you want the flicker as much as the smell, and in a house with drafts and open windows a diffuser holds its scent far better than a flame that gets blown sideways every time a door opens. Check the wick on a candle before you buy it if you can, a thick braided wick in soy or a soy coconut blend will burn evenly and pool properly, while a thin wick in a pure paraffin candle tends to tunnel down the middle and leave half the wax stranded on the sides, which is money sitting in a jar doing nothing. For diffusers, pick the bottle up and tip it, a reed diffuser worth having uses reeds that actually pull the oil up rather than sitting there looking decorative, and the glass should feel properly weighted, not like it would tip over if someone walked past too fast.
Where they stop being worth it is anywhere the smell has to compete. A diffuser in a kitchen fighting last night's garlic is never going to win, and a heavily scented candle in a small bathroom with no extractor fan turns cloying fast. If you have a cat who likes knocking things off shelves, rethink the open flame near anything breakable, and if you or anyone in the house gets headaches from strong fragrance, a diffuser lets you dial the intensity down by using fewer reeds, which a candle simply cannot do.
Avoid putting a strongly scented candle anywhere near where you cook or eat, it genuinely ruins food, and keep scented pieces away from rooms where you already have fresh flowers or an essential oil setup going, because two competing smells in one room just reads as confusion rather than anything pleasant.