Can Darth Emo be an effective Supreme Leader?
Personally, I think he's totally incapable of running an Evil Galactic Empire. He's extremely powerful, but also immature, impulsive, self-absorbed, and not very bright. And he doesn't seem to know how to delegate or organize, or how to get his way by any method other than indimidation, how to inspire loyalty, or deal with bureaucracy. In fact, he's been Snope's attack dog, his head full of whatever Snoke put there, sent out to do jobs other goons can't do, but not trusted with a power structure of his own.
Put him in as Supreme Leader, and after one year the Empire will actually be run by clever bureaucrats who patronize their ignorant "Leader", while he alienates entire system by Force-choking their leaders, and masses of ordinary citizens join the REBELLION. He'll be more insecure and tempermental than ever, because he realizes he can't do the job he's claimed.