This page gives the reasons I argued BQN discussion should move from the APL Orchard (perhaps also known by the shortcut URL apl.chat) to its current forums. As nobody's proposing to move back, the only reason to keep reading would be historical interest.
StackExchange chat requires a StackExchange account with enough reputation, or moderator-granted permissions, to talk, and sets a reputation threshold for some minor features within the chat as well. Those who don't use StackExchange have to make an account and send an email to get permission. While this filtering probably leads to a more dedicated forum base, it does this at the expense of keeping out a lot of people, including many who have a lot to contribute. I think this isn't a good tradeoff, and this seems to be the community concensus as well.
StackExchange is not built for bots, and existing tooling for it is not very good. At the time of writing the bot that used to link the APL Orchard to IRC #apl is able to write but not read posts, so that the link only runs one way.
For a while StackExchange offered significantly better chat features than other forums for discussing code. At least Matrix and TopAnswers are now at parity or better, and have less confusing quirks relative to mainstream forum software like Discord. They also make it easy to create new rooms, which is much harder on StackExchange.
There are concerns about StackExchange moderation: while he has since declared language design discussions to be on topic, Dyalog employee Adám Brudzewsky previously unilaterally changed the topic rules for the forum, over the objections of all commenters at the time and all of the forum's most active users. It's a minor offense, but this indicates to me that the APL Orchard is in practice a Dyalog-controlled forum, so I recommend using it only if that's what you're looking for. Previously members of the community had discussed moving and experimented with TopAnswers (unsuccessfully in my opinion). This event prompted many users to leave and was when the Discord and Matrix BQN forums were created.
In addition to concerns about the APL Orchard specifically, StackExchange chat (along with StackOverflow) is owned by Stack Exchange, Inc. This company is no longer owned by its original creators, and in recent years has operated in a generally unscrupulous way—which has caused employees with ethical concerns to leave or even be fired, a progression that's unlikely to end well. This thread documents these issues. StackExchange chat is insulated from any such issues by being effectively unmaintained, which is hardly an ideal defense for your community's main forum.