'extremely adverse' is correct and usable in written English.
You can use it to describe something that is very unfavorable or damaging. For example: The extremely adverse weather conditions made it impossible to finish the race.
In extremely adverse situations, rose-colored thinking can actually spell disaster.
Self-reported extremely adverse life events and longitudinal changes in five-factor model personality traits in an urban sample.
But the failure to restore basic services, such as electricity and telephone service, in many areas has had an extremely adverse impact on popular attitudes.
Charities, they argued, would lose donations and customers because they would be subject to extremely adverse publicity if their involvement with the programmes was revealed.
It was designed to be controlled by remote login and to operate for the long-term in extremely adverse weather conditions.
From the beachhead at Port San Carlos, the British infantry advanced rapidly southward, through forced marches under extremely adverse weather conditions, to capture the settlements of Darwin and Goose Green.
As an extremely rare adverse effect, even healthy individuals can experience a seizure during TMS.
You'd have to be extremely risk adverse to choose the former.
The most successful entrepreneurs we know are extremely risk adverse.
Effective vaccination campaigns, which succeed in eliminating the corresponding diseases, may result in considering unacceptable even extremely rare adverse effects.
We also excluded one study which used a sequential design [62] and one small Malaysian study which showed a discordant, extremely strong adverse effect, of industrial trans fatty acids [63].
