One of the issues in "The More Complete Cannoneer" reguarding cast-in-place liners is the concentricity of the liner. I don't have any experience with Hern's bigger tubes, but I do have a Hern Coehorn, which is about the same size as the Spanish Howitzer barrel. The liner appears to be pretty much dead center in the bore. I wish I could x-ray it, but they are cracking down on what I sneak into work, and a 92# barrel is hard to hide in my Carhart Jacket pocket.
If anyone was keeping tabs on "Charcoal Burner's" "Mortar in the making", the Hern barrels appear to be at least as sane.
The undrilled vent hole appears to be an attempt at avoiding liablity if anything stupid happens. If I wanted a lawn ornament, I wouldn't have shopped for something with a 1/4 inch steel liner, and a fiberglass make-believe barrel would have cost way less to ship.
For whatever it is worth, Dixie Gun Works drill the vent hole in Hern barrels, resells them, and doubles the price.