I'm a former firefighter and as the OP stated, this thread could take off in a lot of different directions. Trust me, there was never a house fire or a structure fire that I went to that these things were not thought of and looked out for - either by questioning the owner or if no one was present, by careful observation as you entered the structure to fight the fire.
As stated, these containers are not designed to contain an explosion - they are for security. Personally, I have always stored my powder in an outbuilding. I live in a rural area. For years, I kept it stored on a shelf and when I was on the FD, it was known that I had it if there was ever a fire. In the last couple of years, I have gone to storing it in a small heavy wooden box that can be securely locked. Actually, it is more like an antique type tool chest. The box is not marked as to the contents. This is more for my peace of mind should my building ever be broken in to. Yes, they could break in to the chest or even carry the chest away - but at least I have tried to secure it. If someone steals it . . . . well, sometimes you just can't fix stupid.
If I remember correctly, there are some BATF requirements as to dealer storage and also as to the quantity that an individual may possess - but, I may be wrong on that. If a person, like the OP feels a greater "peace of mind" in regards to storing their powder in one of these . . . more power to 'em. I don't personally feel there is a "right or wrong".
As far as "security" - i few these about like I do "gun safes". So many folks buy expensive "gun safes" thinking that they are protected from theft if they store their guns in one (or from fire as well - but, that all depends upon the "fire rating" of the safe). Unless that say e is securely bolted to a steel beam, cement floor, etc. - it is a false sense of security. Yeah, safes may be heavy, but the can be easily moved by someone who knows how to do it.
What it all boils down to . . . regardless of if it is the storage of gun powder, primers, ammunition or propane tanks is "common sense". To put it in perspective that a "muzzle loader shooter" can relate to . . . black powder comes in 1 pound cans. The service load of a Civil War 10 pound cannon (Parrott or Ordinance Rifle) was 1 pound of black powder. That was enough to send a 10 pound projectile a mile and a quarter. That's a lot of potential energy stored in that 1 pound can - so use common sense in where and how your store it. The same goes for your full powder horn.