Plug in diffusers that run off a car's 12 volt outlet look appealing on paper: a device, a reservoir of essential oil or fragrance, powered directly by the car itself. The reality of living with one for a few weeks tends to surface a handful of problems that rarely make it into the marketing copy.
The most obvious one is the outlet itself. Most cars have a limited number of 12 volt ports, often already occupied by a phone charger, a dash cam, or another accessory, which forces a choice between fragrance and something you probably need more. Some diffusers only run while the engine is on, which means the fragrance disappears entirely the moment you park, an odd tradeoff for something meant to make the car pleasant to sit in.
There is also a mechanical component to worry about that a hanging card simply does not have. A fan, a wick, a reservoir that needs refilling and can leak if overfilled or knocked loose while driving. More moving parts means more that can eventually fail, rattle, or need replacing, on top of the ongoing cost of refills.
And the fragrance itself is still, in most cases, a fairly basic formulation, since the device's whole value proposition is the mechanism, not necessarily what is actually inside the reservoir. A clever delivery system built around a mediocre fragrance is still, at the end of the day, a mediocre fragrance.
A hanging perfume skips all of it: no outlet competition, no moving parts, no refill schedule, just a genuinely well built fragrance doing the one job it was designed for.