Real blackpowder differs from Pyrodex in that it ignites at lower temperature, producing more fouling per shot assuming equal volumes of powder are used.
In my opinion, that fouling is softer, easier and quicker to clean, and less corrosive by nature than that left behind by Pyrodex. I think that real blackpowder has a definite advantage over the subs in terms of performance under really cold weather conditions or when the humidity is high, where I believe it provides more consistant, instantaneous, and reliable ignition.
I don't think blackpowder is always more accurate than a sub. With my own arms, I know this simply isn't true. Using my 1:32 barrel to shoot conicals from my .50 Great Plains Rifle, Triple 7 delivered the tightest groups. Let the temperature drop below freezing or the relative humidity top 80%, however, and it doesn't seem to perform as well. It doesn't work well for me at all with patched balls from the 1:60 barrel. My experience with my Pietta 1860 is similar.
Nothing works as well in my Pedersoli 12 bore as the real blackpowder deal. Ignition is "right now" regardless of humidity or temperature. When the shooting is over, the gun gets coated with Bore Butter in and out. When I get home, a totally clean gun is only minutes away. Dittos for my 1861. It shoots the best groups with FFg Goex and .375 balls, which are usually what I am forced to shoot. When I can get the better fitting .380 balls, FFFg Goex and Pyrodex P are about even.
When I weigh it all, the real stuff is pretty hard to beat because it offers consistant performance over a broad range of climatic extremes, has a long shelf life, and cleans up easily and quickly with water and a little Ivory soap, and its cheap if you happen to have a local source of supply.
That said, I like using Pyrodex P and Triple 7 in both FFg and FFFg forms, too. It kinda depends on what I am seeking to do and when. The two that I don't have much use for are Pyrodex RS and American Pioneer because they just don't work well in my little battery of blackpowder arms.
YMMV, and all of that...
-JP