Personally I would advise against in speaking directly to them about your concerns (i.e. you letting them know you think they are a bad teacher) - it is probably the easiest way to fall out of favour with your teacher.
I don't think you give teachers in general enough credit honestly and I say this with the greatest of respect - if it is approached in the correct way it can be quite beneficial to all parties involved. Of course if you go up to them and say "you're crap" then it's going to cause something, but if you perhaps approach them in a nice way and say "look I didn't really understand x or y can you please help me" 99% of the time they're going to
want to try and help you (the 1% don't deserve to be in the role they're in).
I am really sick of people thinking teachers are mind readers - sure you could fork out money and get a tutor but hey, you're right in saying marking of essays is subjective and perhaps doing it in a different way to what the teacher has taught may be counter productive for SACs. Teachers cannot automatically detect that somebody is having difficulty - speak up if you don't get something.
I dunno, from my personal experience I really think it is worth having a private chat with the teacher and discussing that you aren't getting a specific concept and require further assistance. Wording it correctly is important to maintain a professional relationship - after all, teachers are humans as well. You aren't going to like every single teacher you meet and not every teaching style is going to suit you... complaining on ATAR Notes isn't really as effective. The teacher is still there, this isn't going to change anytime soon and they will keep teaching the same way unless you let them know you aren't getting it... because silence is golden from a general teaching point of view (a good teacher would do what's called "check for understanding" at regular intervals so they can identify who gets it, who doesn't and what intervention strategies are necessary for success).
I know this is a blunt response but really I needed to write this one.