The main complaint I have is that some of the electronic items sold by Kogan.com.au are faulty or don't work well (e.g. poor video quality), and their after-sales support isn't great.
I bought a car dash camera from Kogan.com.au which has worked fine during the daytime (not very clear video quality at night). However sometimes the camera doesn't record anything at all except for audio. See for yourself. In addition, I've bought many other electronic items from that website (mostly spy technology) that turned out to be duds. In one case the item arrived damaged and I had to argue to get my money back and even had to email the boss at Kogan himself - Ruslan Kogan.
I only got the refund because I copied Kogan himself in my complaint email as I had met him previously for a coffee while soliciting donations for Liberty Australia.