In English, two negatives make a positive. In some other languages, two negatives emphasise the negative. Thus an international reader may assume that 'not unusual' means 'very unusual'. Negative language is hard to translate and often causes grammatical errors. Negative language is confusing, because it is hard to think about non-existent things.
Negative language has psychological effects that can be damaging in an international context.
Some cultures regard negative language as insulting, embarrassing or shameful. If you must use a negative word, keep that sentence extremely short. The shipment will not arrive until late January. Don't click here. The shipment will arrive in late January.