no not really. If youre really bad at that 1 unit (prolly an ext subject) it is not counted in your atar. the atar calculating ppl only use 10 units to calculate ur atar. the worst one or two units is ignored. however, if youre really good at that one unit, 1 unit of your weakest subject is pushed out of calculation (some cases i think if ur mark is really good then it can completely neglect ur weakest subject), and ext subjects have god-tier scaling so use it to ur advantage.