When considering a subject you choose, I would say picking it based on: your performance ≥ your interest > difficulties of the subject for you > scaling
I think typically a band 6 give a boost to atar regardless of the scaling, so I suggest you to check the honour roll online about the amount of band 6 in engineering from your school each year. Other than that I agree with everything that jimmysmith560 had said, it scales in the upper part between English advance and English standard to give you a sense of scale.
After you understand the atar system (you can find it on youtube from UAC), you would realise that harder subject tends to scale better, so you would normally get a higher mark in low scaling subjects comparably and that would give you an equivalent atar based on your performance regardless of the subject (the very reason that scaling exists)
I think it is also too early for you to say as well since you haven't started it yet, you can also try it in year 11 and drop it in year 12 if you don't like it or is bad at it, but usually, you can only drop 1 subject given that you don't do 13 unit in year 11 and extension 2 on year 12
Sometimes the workload of the subject is also important because you have to balance between the subjects, I suggest you find someone from 12 doing engineering about the course, from what I heard engineering has a heavy workload, so you can ask about it in your school.