While I only load 45 LC with BP, I do love Unique and use it for most of my other calibers in pistol (at least for the lighter to moderate loads).
I use unique in 44 Mag with no issues. Unique does tend to be a bit dirty, but I have never had a problem with the dirt affecting operation of the gun. When I was heavy into pin shooting, I would put maybe 500 rounds thru a Ruger Redhawk every Friday night without any cleaning during the 500 rounds. While my pin loads are more in the "moderate" range, my practice loads were quite light.
There is a minimum amount for each case for which, if you drop below it, you will start to get erratic ignition. If you switch from standard to magnum primers you can drop the charge a little bit more before the erratic ignition happens.