I have used them and still do on occasions in my experience I would never trust a joint made with one that has to stand any stress. If you just want to stick something decorative down its ok. If on the other hand you need to be sure the joint will hold use something else. The more porous the item to be glued the better hot glue will hold it.
Yes the glue joint is waterproof BUT its perfectly possible for water to get under ( capillary action) a joint on a smooth surface and cause a joint to fail.
By now you have probably gathered I am not a fan. Oh and if you do use one watch out the glue is hot enough to cause a burn and it sticks to skin fine.
"that's not a bug its just an undocumented creature."
Sir Terence David John "Terry" Pratchett, OBE (28 April 1948 - 12 March 2015)
I have a lot of experience with Glue Guns having taught Technology in schools for 30 years +. There are two main types of glue stick-Hot melt and Low melt (for safer use). The hot glue will stick skin with often devastating results!!. I also build theatrical props and it is very useful for building all sorts of things and will stick most things well. ie wood, metal, plastics (which it can effect) glass, clothing, felt, straw, string, rubber and many many more, but they are all quick fixes and can come apart with constant use and vibration especially on shiny surfaces. In models I tend to use the stuff to hold cables in place and tack things in position quickly to align them before finally gluing with Epoxy. Motors can be be held in place with Hot Glue and the advantages are it makes a good sound insulator and can be repositioned for a few minutes before it goes cool. Also it can be prised off again if you need to change the motor.
I would not not recommend it as a permanent fix on many model things and certainly not as a structural adhesive, fittings etc. will almost certainly fall off eventually. If you regard it as a third hand you'll find it a useful addition to your gluing arsenal.