The slash commands are diverse and represent features that may not be implemented or ready just yet. For example, you can /chan_join a channel, but /chan_invite at the moment doesn't work. (I imagine chan_invite has some difficulty: namely the functionality of the client inviting someone offline to a channel the next time they login isn't easy to reproduce. And it's something that XMPP doesn't do on it's own.)
Like the UI windows, you can try a slash command and if you get "unknown command" as a response, you know it isn't "ready" yet. ...Or "ever" on some commands at SCoRE's option. I don't work with/near them, so I can't answer what is and isn't going to come to pass. The
roadmap offers clues to some slash commands that might be considered as part of the future use of PChat, but like most F/LOSS the caveat is that there is no timeline or guarantees, it's ready when it's ready.
Some of the slash commands do have quirks: for example, calling a costume change emote is different in PChat than it is in-game: "/emote ccbackflip" works versus the "Official" way of doing it with "/cc_emote 3 ccbackflip". (Also, the Costume UI in game doesn't have the CC Emote drop down list that the live client had, so selecting them that way doesn't work either.)