Cant' answer the question either but know from experience that Milners with a drill pin inside the collar is much more difficult to blow successfully even with Gelignite (1875 onwards) as the charging instruments have much less room in which to operate. Before that time the Gunpowder-Proof design prevented all but a thimbleful of the powder charge to be introduced which was totally ineffective, even when placed over the top of the collar.
The thinner wall of the pipe key also had a durability disadvantages.