The problem is that seemingly small errors all potentially add up. A meter a third of a stop off... a shutter speed slightly off ... a gray card half a stop off.... pretty soon you might be way out of bounds for a decent chrome exposure, or even a precise negative.
Light bulbs themselves tend to be misunderstood. The kind people buy at typical retail outlets are all basically outsourced trash, and often deceptively labeled. Traditional blackbody tungsten bulbs are now nearly gone. GE is completely out of the bulb mfg business now, although remnants still turn up for sale. They had two completely separate divisions, one being commercial and scientific lighting, with high quality control and good specifications available, the other the Consumer division, geared to high volume with low quality, and full of unreliable marketing BS. That's the way it is with some other companies too. It applies even to enlarger bulbs.
As far as gray cards go, Wilt, I don't care to repeat the whole story. I did full spectrum plots all the way from IR to UV with a FAR more accurate gear than you mentioned. Discovering the sheer lack of acceptable quality control in most brands of gray cards can be seen with the naked eye; but I wanted to quantitatively pin it down. Many were at least half a stop off. But that was just a adjunct project to my main goal, which was to make my own truly neutral 18% gray paint - a trickier task than one might think (for example, no commercial black pigment is in fact neutral black, or white pigment entirely white). But that's a whole other story.