As of right now, when I join a game and disguise...it works perfectly fine, and as of yet, I've yet to see anyone who hasn't been fooled by the disguise, or at least nobody made it too obvious.
However, I do have to point out some things I've noticed that will quickly be picked up be others:
As I am of the friend rank, my joining the game message appears in all of it's pinkness. It's only a matter of time before people realize that, when a pink name joins, then leaves before the game starts, the friend is still there. This is especially apparent due to the fact that it says I leave at the same exact time as my alias joins, giving the idea I've been "kicked" by a lesser rank. Diamond donors I assume also have this problem, but at least they don't have a name that stands out as much as friend. (Diamond donors are pretty common these days in comparison to before. I almost don't even notice them anymore. (Sorry, lol.)) It might be a bit much to ask, but I would like if friends had silent joining and no "leaving" when using /disguise. It's only a matter of time before people figure out that "friend leaving server"/"Friend not in server when it said they joined" = "friend is person who joined directly after friend left"/"Friend is still in here".
As well, though I haven't yet tested this out...can't someone just see you turn into somebody else if you are in front of them? This is kind of annoying if someone is following you around. (I almost never have this problem myself, but I can understand the frustration of this for youtubers.) This is mostly only a problem if the command isn't typed straight away upon joining the lobby.
These two things aside: I'm glad this was added. I finally feel like I can play proper games without being on my alt. Once everything abuse related is solved, even if the command doesn't fool everyone, I'm at least glad it's here. Still, it's only a matter of time before the majority of people figure out the things I said above if they're not solved. (Mostly the one relating to joining/leaving for friends.)