Hot glue mostly just forms a mechanical join to whatever surface it is applied to. The heat "might" create a key on some materials (by locally melting the surface), others might need a good coarse abrading. If the plastic that is being joined has similar thermal characteristics to the "glue" (which is really just a stick of plastic) you might end up with a welded join, with pretty much the same perils as welding metal, but at a lower temperature.
It does tend to be messy, readily forming strings, and tends to look like something that Fraggles would look down on with distain, but every so often it is "the" answer. But certainly nothing like as often as it was originally touted to be when it first appeared.