Outdoors, a sufficient distance from flammables and buildings that a mixer ignition can simply be stepped back from with no consequential damages, except the 2nd and 3rd degree burns of the mix operators and any broken bones from being blown backward.
Burns are considered the least comfortable injuries possible.
I have been friends with two people with mix accident burns. They very strongly recommend against taking those risks at all, much less the typical behavior of "TRA EX" guys.
I for one wish amateur rocketry was engaged in by prople with "professional techniques" and none of the fringes of consumer rocketry included mix kits and mixing practices at all.
There is something to be said for motor design - from precast propellant sticks.
Order them from me and save yourself a life ruinous experience.