It depends on what music you put on your build.
For instance, I have the RWBY song "This Will Be the Day" on my build, Final Destination and Pokemon Stadium 2. Whenever a match is recorded and the song is played, and subsequently uploaded to Youtube, I get copyright notifications on the song.
I don't get my videos taken down. More like I get ads on my videos that do not go to me, but rather the person that owns the copyright for the song (I do not monetize my videos). So things like that could potentially happen if you have copyrighted material in your music build.
There are two solutions for the issue. One, you could simply NOT have copyrighted music on your build. That won't get you any strikes with anyone. And if you do not monetize your videos, nothing significantly bad will happen to you.
Two, you can keep the songs on your build, but when you are recording, turn off the music at My Music. The beauty of Brawl is that you could control what music plays and how frequently. If there's a song you really like, but are uncertain of the legal ramifications of said song, you can always turn it off to avoid it entirely. It's not like Youtube can detect you have such a song on your build anyways.
