I'm really the only Mu player in the local Pacific Northwest Scene, from what I can tell, and I just started taking baby steps into top-end play. As a result I get quite a few people looking to play me for matchup experience in casuals.
I've definitely taken steps to become less predictable in general from playing him. I feel like there's gaps and easily punishable problems with my gameplay, yet that's not what I hear from feedback in casuals. The biggest issue I keep dying to is that I feel conditioned to attack in a certain way, one that gets me killed. I'm not sure how to avoid this, though I started breaking away from that in the later matches. I feel like I have "sticky fingers" most of the time, and working out the rough patches will definitely help.
The one bit I did learn from him, when we talked about the matches, was that while Hakumen can deal with Steins, he can't actually punish them. The matches I did win where very dependent on jumping out > j.5D > airdash away after a failed blocksting. The stein tracks Hakumen if he IAD's to follow, and you're out of range of Hotaru if you space it right.
Edit: Sorry, I edited this like nine times while you might've been posting, it just never came out right.