Sort answer: Yes, the hub is safe to use.
Long answer: As with anything on the internet take it with a grain of salt. Contrary to popular forum belief, not everyone that buys a DJI drone is a member here to document there experiences, hell some people here don't post much at all. There's good and bad with any product, not many people sit here and post the good, most times we only hear the bad.
DJI did not sell a couple of
MA2 FM combos and had every battery go bad because of the hub, a few people have had a few bad experiences and want to make sure the world knows. I for one, initially charged all of my batteries using the hub and have gone between the hub and singe charging since then. I have had zero issues.
Forum logic is one person's problem account for thousands of failures, so get ten people with the same problem and boom, armchair engineers unite! Don't worry to much about it, use the hub, single charge, chances are your batteries will be fine either way.