It is going to depend on your students. Generally, the hard parts are exactly what you expect.
If your kids are not well prepared in math then 1.4 and 1.5 on binary plus there is a little piece on modulo arithmetic. Related to that are conditionals introduced in 4.6. That will include the infamous
a == 5 || 6 mistakes.
Event driven programming is introduced in 3.6.3 without much explanation. That will haunt you the rest of the year.1
a = [getColumn()] mistake rears it’s head due to that confusion.
Unit 7 Parameters and
return are introduced after we have taught the kids how to avoid using those concepts. They don’t see the need to use them so you need to take the time to show them.
1It should be noted that while App Lab introduces callbacks without explanation the most popular alternative, Scratch, introduces multi-threading and concurrency without explanation.