It's like the problem with simple deduction in philosophy. Yes, if your premises are right, valid deductions will lead you to true conclusions, but the problem is knowing whether the premises used by the old metaphysicians (or modern ones, for that matter) are true.

I suspect that using only valid deductions, while manipulating terms that already have real meanings attached to them, probably poses at least as great a problem as avoiding untrue premises.

I remember during a logic class I took, the teacher made an error of deduction, and I called her out on it. She insisted that it was correct, and every other student in the class agreed. I tried to explain the mistake to her after class, and wasn't able to get her to see the error until I drew a diagram to explain it.

It was only an introductory level class, but I don't get the impression that most practicing philosophers are at a higher standard.