Yes getting the units wrong is said to be an issue. In the UK when the altimeter setting is passed over the radio, if the number is less than 1000, the units have to be said. Thus: "QNH 990 hectopascals". It is said that the reason for this rule is that US pilots flying here might mistake "QNH 990" for an altimeter setting of 29.90 inches and thus set their altimeter wrongly. Regretably a large proportion of pilots and ATCOs find it hard to pronounce "hectopascal"
