What is the most difficult line cook position?
I would say the oven.
The oven handles both apps and entrees, with cooking times for those items ranging from very quick to quite some time. Ovens have hotter and cooler areas so you usually have to shuffle things around while cooking. Ovens fill up on busy nights making it difficult to manage all the things inside, and you can't monitor how things are cooking visually unless you're constantly opening the doors (which is obviously not a good thing to be doing), plus you can't just turn up the heat on something if it's not cooking or browning properly...you're just at the mercy of the beast, having to keep a lot of mental track of what went in when.
Also it's hard on the hands, lifting food-laden sizzle plates with tongs all night.