Travellers pack an adaptor and assume the electrical problem is solved. Plug shape and voltage are separate issues, and only one of them is fixed by a piece of moulded plastic.
An adaptor changes shape and nothing else
A travel adaptor rearranges pins so a plug fits a foreign socket. It passes the supply through unchanged, at whatever voltage and frequency the building provides.
If the device cannot accept that supply, the adaptor has simply connected it to something harmful more conveniently. The fit is not an indication of compatibility.
A converter or transformer is the device that changes voltage, and it is a different, heavier and more expensive object than an adaptor.
Most modern electronics are already dual voltage
Laptop bricks, phone chargers and camera chargers are typically built to accept a wide input range covering both common world standards, plus either mains frequency.
The proof is printed on the unit itself, in small text near the plug. An input range spanning roughly one hundred to two hundred and forty volts means no converter is needed.
Anything showing a single narrow input range is designed for one region only. Those are the devices that fail abroad, and the failure is usually immediate.
Heating appliances are the persistent problem
Hair dryers, travel kettles, irons and styling tools draw far more power than electronics and are frequently built for one voltage standard alone.
They are also the devices most likely to exceed a small converter's rating. A converter sized for a phone charger will not carry a hair dryer, and overloading it is a fire risk.
This is why hotels supply dryers and why a dual-voltage version is worth buying if one is genuinely needed rather than improvising with a converter.
Earthing and socket design vary as much as voltage
Some plug standards include an earth pin and others do not, and an adaptor that omits the earth connection leaves an appliance unearthed even though it works.
For a laptop charger this is often tolerable; for anything metal-bodied it is not. Universal adaptors differ considerably in whether they carry earth through properly.
Socket shuttering, recess depth and pin spacing also differ, which is why a cheap adaptor can sit loosely in a socket and arc under load.
Frequency matters for a shrinking set of devices
Mains frequency differs between regions, and equipment with a mains-driven motor or clock can run at the wrong speed even at the correct voltage.
Modern switched-mode power supplies are indifferent to this, which is why the issue has largely disappeared from consumer electronics.
It survives in older appliances and some specialist equipment, so anything with a mechanical timer or a synchronous motor deserves a check before it is carried abroad.