Burn rate is not irrelevant, but it is only one of several factors that must be considered. For autoloaders it is necessary to put enough powder in a case so that upon firing, the bullet exits the barrel and the resulting reverse impulse of the the slide ejects the empty case and picks up & chambers a fresh round.
This must all occur within the SAAMI pressure range, which should not exceed about 16,000 psi for standard pressure .45 ACP loads. This is very easy to accomplish but the devil is in the details. If you have a 16,000 psi reload that functions at 100%, but the bullet exits the barrel at 400 fps, this will be unacceptable for most reloaders. Ditto if you get 850 fps but the load won't group any better than 12 inches at 25 yards.
This is where the burn rate comes into play. You could use a verrry slow pistol powder like Alliant 2400, but you can't get enough 2400 inside a .45 ACP case to make a very good factory duplication round. There's just not enough case volume in the .45 ACP for 2400 to work well. Velocities will be low even if accuracy is decent.
The reverse can also cause problems. If you use a light load of a very fast burning powder then the powder flops around inside the case and the inconsistent powder positioning can cause poor accuracy (although Alliant Bullseye is a fast powder that generally produces good accuracy).
Most reloaders use a powder charge that fills most of the case volume underneath the seated bullet while producing near-factory velocities and decent accuracy. That's one reason that Alliant's Unique is such a great powder for the .45 ACP round. It meets all the above criteria for accuracy & velocity, and as such is proven in this caliber. Lots of other powders will perform similarly, such as WW231, Vitavhouri N320, etc. This is why experimentation is necessary to determine what the optimal load is for YOUR gun and your personal shooting style.
-Lee