I don't use MC a significant amount of my time, but when I do the hardest part is often debugging my work.
Here MC falls short - the error messages are cryptic, at best, and misleading most of the time. Solving multiple equations and unknowns with the Given ... Find method, often the trace error wasn't even close the real problem once I find the issue.
This makes MC less useful to me and I don't use it when I have a close deadline and need to account for an inordinate amount of debugging time. Tell me if I have an undefined variable, or a units mismatch, or whatever - at least the general category. Obtuse error messages, or worse saying something is wrong in an equation that isn't even close to the error area, makes this tool less useful to me.