You get to make it compliment your specific gun perfectly, vs. That's part of the advantage of making your own ammo. Guns work best when all the parts compliment each other. It might be a squib, or is might just be really inaccurate ammo, etc.

Instead of flash from unburned powder what you get is all the powder consumed with the bullet only part way down the barrel. Now, just like using ammo designed for long barrels in short barrels gives less then optimal performance the same is true if you use ammo for short barrels in a long barrel. Since suppressors work best with subsonics that extra barrel length is just unnecessary. As you understand though the advantage of a long barrel is higher velocity potential. Now, for either the short or the long barrel the powder selection & charge weight can be tailored to produce subsonic velocities. But if we load a fast burning powder that is mostly consumed by the time the bullet leaves the shorter barrel you get less flash & noise. So you get a lot of muzzle flash & noise, and I would imagine not very good suppression. If you shoot this same ammo in a short barrel much of the powder will be burning after the bullet leaves the barrel. If you had a long barrel you'd tailor your ammo with a slow burning powder so as to take maximum advantage. You have to start short just to end up medium length.Д¬onsider if you were making your own ammo. There's also the practical issue of a suppressor adds a lot of length.

Subsonic rounds without suppressor