Main idea: Flying units will start to fly up very slowly over an obstacle, and down very slowly, taking about 30 seconds to clear something that would have taken 5 had they gone around it. They will often change elevation despite being easily able to clear the structure at their current height- or worse flying down to match the dip in the floor. This pathing pain is exacerbated by how badly the speed of flight decreases during elevation changes makes the unit control and feel. Pits with no floor also make them do wonky things, pathing randomly to nowhere before following orders.
Further details: In battles like skaven settlements there's commonly several levels and rocky outcroppings. If you have a lord on a dragon and tell them to fly straight, if there's a floorless pit in their way they will often turn around and fly nonsensically backwards in a circle for a bit then start to fly forwards. When they get to a hill they will rise at about 10% of normal flight speed until they hit the top, fly straight for a few paces then start to descend at the same glacial pace. They will change elevation constantly to reflect the changes of the ground elevation even though it comes nowhere near to touching them. I had infantry units who were given the same move order beat my lord on a dragon to the other end of the map. I think that tells you exactly what you need to know.