Technically, wrist and upper arm blood pressure monitors are identical. The accuracy in the laboratory is exactly the same (+/- 3%). It is therefore not possible to say that one type of blood pressure monitor is more or less reliable than the other.
The choice of a blood pressure monitor is actually based on the user's profile.
An upper arm blood pressure monitor is recommended for anyone wanting to monitor their blood pressure when they do not have a physical (arm too big or trapezoidal shape) or medical contraindication.
A wrist blood pressure monitor will rather be advised for people with a high arm circumference (>42cm) or a trapezoidal shape. For these people, taking blood pressure at the wrist will be more accurate because the radial artery is closer to the surface of the skin (signal captured more easily).
Conversely, wrist blood pressure monitors are not recommended for elderly persons who may have "weakened" radial arteries (arteriosclerosis) or are prone to tremors.
It should be noted that a wrist blood pressure monitor requires stricter respect of the precautions for use to guarantee the most reliable results possible (in particular, to keep the device at the level of the heart and remain perfectly still during the measurement).