Knowing which font is used with a hundred percent certainty, maybe not. This probably is something more people should be aware of. But the failure mode might be some slightly awkward line breaks, assuming you allow for the edge case of alternative fonts. And even when the calculation isn’t a perfect representation of the rendered reality, the approximate width is still more useful for layout purposes than going in blind.

Combined with em-based sizing I’ve found this to be pretty robust. Just because you can’t make it perfect for all users doesn't mean you can’t do the work to improve it for many.