It's not that complicated although it might seem so. I'll keep it silly simple.
Myostatin is produced by the body and limits muscle growth. The idea behind a Myotstatin Inhibitor would be to stop the limiting of growth. It'd take very little stimulus to illicit a reaction by the body to make muscle grow. Now if you took it (assuming it worked) and trained your ass off you'd grow... and grow.
Check out the double muscled Belgian Blue Cattle.
It sounds great. But isn't. The in-breeding of the cattle means their hearts are fucked and they have much shorter life compared to other breeds. Like humans would also get they have problems with their joints and connective tissue.
So if it was legit here's what would happen - muscle and tendon tears. Heart and cardio-vascular issues. In other words muscle growth would out strip the bodies ability to deal with it. It'd be like a fairy godmother adding 40lbs with a wave of a wand. You'd spend months adapting and would incur injuries.
And, as above, you need a selective version. As it stands no such drug comes close. Get used to adding 3-6lbs a year but do that for YEARS. That's all I ever did. I went from (aged 18, having started age 15) 175lbs to a recent high of 318lbs. Again as previously noted I'm a strength athlete not a bodybuilder (so no abs) and I'm 6'3