I'm a late-comer to this party, but I'll try and throw in some data.
Unique and Universal are not interchangeable, but I have used Universal in place of Unique in 240 grain jacketed and lead loads. What I was doing was using Unique and loading 44 SPL loads in 44 Mag cases. At that level, neither Universal or Unique are going to blow up the gun, but the velocities were different. The idea was to have nice soft practice loads and both powders could do that just fine.
I've since used Universal in 357 Mag in place of Unique. Same story: I was using a 38 SPL recipe in 357 Mag cases. It works. My EDC load uses Universal-- again, I started with a 38 SPL load for 125 grain Hornady XTPS and just worked up until I was about half-way between 38 SPL velocities and 357 Mag. The recoil is greatly reduced, but the velocity is much better than 38 SPL. Lead Lee 358-125-SWC fire nearly identically, so I have a cheap practice load.
