Long/complex Excel formulae should usually be broken into smaller chunks, with intermediate results stored in separate cells. For example, if you're calculating two numbers, and then calculating their ratio, it would be better to use three formulae instead of one. That way:
1. Each formula is shorter.
2. Each formula has a single purpose which can be understood.
3. The outputs of the intermediate steps in the calculation are obvious.
This applies also to complex nested IF() statements. Instead of calculating all the conditions inside the IF(), calculate them outside, and then reference the TRUE/FALSE cell values in your IF statement. When you need to debug why you're not getting the result you want, you can easily look at the intermediate calculations all at once, without needing to step through the calculation and check it in the order of calculation.
Especially since pretty much every year we hear another story where some small Excell formula bug cost company millions of dollars. Redesining Excel formula pane should be their number 1 feature in 'todo'.
...unless they still care to add big features at all, and they aren't just in maintenance mode like most of their stuff