correction:: the only hitstun edits to project m is the removal of attack cancelling out of hitstun and edits to MC window
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Project: M has brawl hitstun? Wow. Momentum canceling really makes a huge difference.
Also, iirc we have some troubles editing event matches and some of the 1-player modes. Also, we currently can't add characters, though that is being worked on, though we can add stages. Stages are fully hackable except for hazards, from what I understand we can't figure out how to edit the damage or knockback. I'm also fairly certain we can't add articles, though with OpenSA badly needing to be updated I wouldn't be surprised if someone had found out how. I believe Tabuu can edit articles.
Essentially, get a hold of BrawlBox, Project Smash Attacks, the latest version of Tabuu, your custom song maker of choice (I use the somewhat outdated BCSM-GUI that's lying around somewhere), and find a few tools for textures and vertexes (I can't help you there, ask around or check the directory) and you'll be set. It takes some messing around with them to get used to them.