The phrase "extremely treacherous" is correct and can be used in written English.
It is commonly used to describe something that is very dangerous, deceptive, or untrustworthy. Example: The hikers were warned about the extremely treacherous conditions on the mountain, but they decided to continue their journey despite the risks.
The feature is extremely treacherous to unwary travelers.
Health officials say the dump site is extremely treacherous.
The National Weather Service said travel conditions in those areas would be "extremely treacherous" by Saturday morning.
You're going to be here a while — traversing that wall that President Trump built between America and Canada seems extremely treacherous.
You're going to be here a while traversing that wall that President Trump built between America and Canada seems extremely treacherous.
He said later that he needed several laps after each restart to get back to full speed on the racetrack, which had been resealed before the race and became extremely treacherous as today's race went on.
This was both extremely flattering for Andriasian, who is rated in the mid 2500s more than 160 points less than the average of the others, and potentially exceedingly treacherous.
Northeast of P'ohong-dong along the South Korean line the terrain was especially treacherous, and movement in the region was extremely difficult.
Many mountain roads, often treacherous at the best of times, remain blocked by landslides, making it extremely difficult for supply trucks to get to the higher Himalayan foothills.
The economy of contemporary music is pretty treacherous and the fact that I can sail through it, well, I'm extremely grateful to people who are interested in my music.
After rattling through cobbled sections and battling perilous crosswinds in an extremely nervous opening block of racing this year, the overall contenders will be relieved that the first week will be much less treacherous.
