The consultant who posted this question is doing something rare: thinking about the people who will inherit the model. That alone puts them ahead of most forecast builders. But their list of formulas to avoid, nested IFs, SUMPRODUCT, INDIRECT, is a good start that stops short of the real problem. The issue is not the formulas themselves. It is whether anyone left behind can trace the logic, change the assumptions, and trust the output without calling you.
The formulas they flagged are dangerous for the same reason: they hide dependencies. Nested IFs create a decision tree that lives inside a single cell, impossible to audit without expanding it line by line. SUMPRODUCT is a compact way to multiply and sum arrays, but it turns two or three simple columns into a black-box calculation. INDIRECT is the worst offender because it breaks the one thing that makes spreadsheets navigable: cell references you can click. When someone sees `=INDIRECT("Sheet2!B"&ROW())`, they cannot trace the precedent. They have to reverse-engineer the string. That is not intermediate-friendly. It is hostile to anyone who did not write it.
But here is the point the original list misses: complexity is not just about which function you use. It is about how you structure the model. A single XLOOKUP is fine. A hundred XLOOKUPs pointing at different tables, with hardcoded sheet names and no documentation, is a nightmare. The real no-no list should include anything that makes the model fragile: hardcoded dates buried in formulas, merged cells that break sorting, manual overrides that are not flagged, and any calculation that depends on the order of rows. These are not formula problems. They are design problems. And they are far more common than a well-placed SUMPRODUCT.
If you want people to use your forecast after you leave, build it like you are handing it to someone who has never seen it before, because that is exactly what will happen. Use helper columns to break multi-step logic into visible pieces. Name your ranges. Put assumptions on a single sheet with clear labels. And yes, skip the nested IFs and INDIRECT. But do not mistake removing three functions for making a model accessible. Accessibility means that six months from now, a junior analyst can open the file, change a growth rate, and see the forecast update without calling anyone. That is the test. Pass that, and your model will outlast your contract.