Is the warning intended for any rifle or just for the Encore?
If it is specifically for the Encore then it more than likely means something and should probably be heeded.
If it is for any rifle (or firearm) then I would guess it is a CYA warning inspired by fear of liability lawsuits and written by the company lawyers.
I'm not shooting 777 any more for reasons that have nothing to do with performance, but when I was using it I used the powder because my rifle gets it's best accuracy at around 115 grains of 777 and I can't get that measurement with the pellets.
I have fired as high as 150 grains of loose 777 3f with a lot of kick and really lousy groups. But, I shoot a Savage and it is designed for smokeless so I don't have to worry about exceeding safe pressures with 777 or Pyrodex.
I only know one guy personally who uses an Encore and he regularily (at least a couple of times a month) target shoots using three 50 grain pellets of 777. When I talked to him a month or so ago he still had all his fingers, eyes, ears, etc.
Mel