I make Starbucks VIA instant coffee at home and it's quite good. If I make it at work it's horrible so I'd say water quality is a big factor. At both home and work I used tap water both from the same city source.

For the airplane I wonder if the water isn't getting hot enough due to lower atmospheric pressure. The flight attendant see it boiling so they assume boiling means hot but it may only be 80C not 100C.

I know when I make the Starbucks VIA I make sure the water is boiling well (I live at sea level) and it's really hot otherwise it doesn't work out.