Lots of people use Excel for various types of math, and it works fine, if you know what you're doing. Excel not being idiot-proof doesn't mean it's not usable.
And that's all without writing a single line of VBA.
(We can have a separate discussion about some of the newer Excel features, like automatically suggesting what kind of statistical analyses to perform on your data - I consider this to be a potential future source of serious fuckups, as it allows people to easily transform data using methods, whose assumptions and implications they do not understand.)
> "looks fine" until it shows a result someone doesn't want, and then an error is found that changes results and the cycle repeats
That's a feature, though. Excel is interactive, which means you get ample opportunity to do sanity checks on results (both partial and final) as you work on your sheet, as well as immediate feedback on corrections. In "properly written" IT systems, this is rarely the case - you end up discovering a problem further down the chain, and have to figure out which component did something wrong, and why.