Many think that cogito ergo sum somehow says or at least implies something about the nature of existence, when it in fact does not. So in that sense, it's not the "big hitter it's made out to be," but that's not a failure of the principle, but a failure of people to understand what it in fact says, or more precisely, does not say.

I suspect that the problem is that when people consider "I think, therefore I am," they think that that "I" refers to the entirety of their self-image, and therefore says that the entirety of their self-image, in all its details, objectively exists.

That's very much not what it means or even implies. It never did and was never intended to stipulate anything at all about the nature of this entity I call "I." Not one single thing. All it ever said or intended to say was simply that whatever it is that "I" am, "I" self evidently exist, as demonstrated by the fact that "I" - whatever "I" might be - think I do.

It's not a coincidence that Descartes himself formulated the original version of the brain-in-a-vat - the "evil demon." He was not simply aware of the sorts of possibilities you mention - of the ramifications of the fact that we exist behind a veil of perception - he actually originated much of the thinking on that very topic. He was a pioneer in that exact field.

Cogito ergo sum doesn't fail to account for those sorts of possibilities - it was explicitly formulated with those sorts of possibilities not only in mind, but at the forefront. And that's exactly why it only stipulates the one and only thing that an individual can know for certain - that some entity that I think of as "I" self evidently exists, as demonstrated by the simple fact that "I" think I do, since if "I" didn't exist, there would be no "I" thinking I do.

And more to the point, that's exactly why it very deliberately says absolutely nothing about the nature of that existence.