Gunpowder, Roman Fire, and other things of that sort were being used around that time, especially if the armies were based in the Orient. It not that far a leap in disbelief.
Make-up wise, what the hell do you want? Thats what the character looks like in the GN. And it's a guy in make-up, not some funky CG creation, so you onlu have so much to add.
As for anything about this movie that could suck, to me it is FAN'S with overinflated exspectations being dissapointed because it wasn't the 2nd coming of (insert religious savior or favorite comic book film here). Just remember, it's based on the GN, so simply just expect a translation of what you read there.