Health threat? Isn't that an exaggeration? Well, it depends. For the average consumer, it probably ain't a problem. With the ever-increasing number of studies that show that berberine could be used instead of metformin and/or on top of medical treatments for high blood glucose and/or lipids, some consumers may end with lower/higher levels of glucose or lipids in the blood when they (a) switch from one 500mg product to the other or (b) simply start a new bottle that was produced by the same manufacturer, but contains more/less of the active ingredient (note: I personally was afraid that the deviations from the label may be much larger and am relatively happy with the results, but I guess that's my personal disillusionment).
The last-mentioned possibility that the potency varies not just between products, but also from batch to batch, is also why I wouldn't rely too much on the data from the study at hand. The next batch of supplement #9, the "best" supplement in this study, of which the authors rightly point out that it was limited "[p]rimarily" by the fact that they "analyzed the berberine content of one individual bottle from each manufacturer" (Funk 2017), may well contain significantly lower amounts of berberine than the one that was tested by Funk et al. for the study at hand. As previously hinted at, this is a general problem with dietary supplements. It's a problem you should know about, and one that disqualifies them as 1:1 replacement for medications, but not one that should make you shy away from supplements altogether, in my humble opinion.