I think the little bit of residue left after concentrates could be just waxes and small plant matter from the concentrate. If you do dabs, do you not have to clean your nail every now and again and get the charred plant material off it? Unless you know the people making it and are doing super filter/butane -> ISO polar methods -> vacuum chamber, maybe rerinse in ISO and dried again type of concentrate making, there is always going to be some impurities. I find my dabs last forever in it too, even on 4 which gives bigger clouds, but understand, it's not a dab nail. You will never get a nail-type hit from any vap. Mind you the nail hits usually give me snot bubbles from 1 hit and 1 hit in my Haze will last me 20 minutes without coughing till tears run down my face, so it's a different experience, but at the end of the 20 minutes, my head is fuzzy, so I'm good. I find also vaping concentrates really brings out the CBD's of the strain and you end up more medicated than stoned. Dab nail, i'm stoned. All in how you use it and what you expect from it.