In essence I have an array of positions and want to punch holes in a plate at those positions.

I am trying with the following:

function pcb () {
    pcb_width = 25;
    pcb_height = 3;
    pcb_thickness = 50;
    pcb_dimmensions = [(pcb_width), (pcb_thickness), (pcb_height)];
    
    return cube(pcb_dimmensions);
}

function holes1 () {
    var array = [];
    array.push(translate([10,10,0],
                    cylinder({h: 20})));
    array.push(translate([5,10,0],
                    cylinder({h: 20})));
    return array;
}

function holes2 () {
    var holes = [];
    var positions = [
        [4, 12, -1],
        [8, 12, -1],
        [8, 18, -1],
        [12, 18, -1],
        ];
    
    for (i=0; i <= positions.length; i++) {
        holes.push(translate(positions[i],
                cylinder({h: 10})))
    }
    return holes
}

function main () {
    //return holes2 ();
    //why does return [pcb()] work but not return [holes1(), or holes2()] ?
    //return [pcb(),holes1()] //does not work
    //return difference (pcb(),holes1());      // does not works
    return union (pcb(),holes1());      //works
    //return union (pcb(),holes2());    //does not work
}

Questions:

1. Why can I not return an array consisting of the pcb and the holes-array (cylinders)? Is nesting a problem here?
2. Union of pcb() and holes1() works, but not if holes2() is used. The function which includes a for loop.
3. Difference of pcb() and holes() works for neither.

As a side note, I am assuming the community hangs here and not on IRC or such?
Looking for a place to quickly solve some doubts to help me get on my feet.