Whether your topic is simple or complex, you still need to lead with your point. A complex analysis might have many sub-points that you prove-up in subsequent paragraphs, but your ultimate conclusion still needs to come at the beginning, and each supporting paragraph still needs to lead with the sub-point you're proving.

Say you're writing a memo to your boss explaining why you need to double the size of your engineering team. That point has to come first. Now, maybe the reason you need to double the size of your engineering team is that management wants to add new product lines, and customers are demanding more customized solutions. Okay, so each of those points are the leads in their own paragraphs.

Where a lot of people have problems is that their writing is a chronological recounting of their thinking about the issue. So it starts somewhere in the middle where they encountered some part of the overall issue, then backs up to where they recognize the larger issue, and then buries the solution at the end.

The structure you were taught in high-school: topic sentence, supporting sentences, conclusion, is simple and appropriate for business communications. There are some writers who can clearly convey complicated thoughts using a more narrative structure, but you'll rarely go astray sticking to that basic style.