You are correct in saying that the metal will spall, but if your only using it as a temporary stove, you will be able to get many good fires out of it (probably years) before it would need to be decommissioned.
If you were to insulate it to make it burn even hotter (and cleaner), the spalling rate would probably more than triple.
When I made one of these about 5 years ago, I was following the 1:2:3 rule where 1 is the feed tube height, 2 (twice the length of the feed tube) is the burn chamber length and 3 (triple the length of the feed tube) is the heat riser height. Over the years this formula was modified, especially when many people started posting their creations on You Tube.
It short, its not that hard to mess up with one of these stoves just to burn some wood. The main thing is to keep the riser much taller than the feed tube so that the fire goes in the right direction.
FYI - It has been said that sometimes the 45 degree angle on the feed tube models has a harder time self feeding long pieces of wood smoothly without getting caught up. However, if your going to constantly be there to tend it or want to use pellets, then probably not an issue.
Feel free to post some pictures of your creation when you get there!