I think that's a vastly overblown (bordering on invented) concern. Not saying it has NEVER happened, but there are all sorts of loads with tons of extra case capacity that don't cause problems. I shoot a .220 Swift load with a 35gr Varmint Grenade that's ridiculously low. As a percentage there are many pistol loads that have a ton of extra case capacity. Ever pull a bullet from a .22LR? They're practically empty.
However, I also think that the closer you get to a 100% case capacity the better it is for all sorts of reasons besides safety.
In my mind consistency is why using Trail Boss (Cheerios) is so good for making centerfire subs. It's pressure to volume ratio is among the lowest, and you can fill the case much more with lower pressures. I feel like it's the easy button, that will give you tight sub groups.
In a situation where I couldn't get any more powder I have a ton of shotgun powder that I can make work. It'll be very low case volume, and probably unacceptably inconsistent, but I can make almost anything go bang with it, and because it's so fast and the loads are so small it will go a long way. Definitely something I would only do as a last resort, but it's there.