I haven't really used any kind of messenger service since probably MSN Messenger and IRC back in the day so I'm a bit behind on a lot of the basics. Part of what's quite different now than the experience then is what modern messenger protocols seem to be used for, as in they have public channels dedicated to topics that function like communities, whereas I only really had experience using them for talking to people I personally knew IRL and manually adding some kind of username to establish talking.
I just got a matrix client and joined a community on a specific interest because I had a question I wanted to ask. I did something similar about a year ago on Discord. This worked.... sorta but the problem I had doing this on Discord is kind of what I think I'm going to run in to on Matrix. If the community is open to the public, there's going to be a lot of people some of whom will log on at different times. If I post a message asking a question hoping someone will have an answer for me, I feel like it's going to be hard to see anybody replying to me specifically because presumably there's going to be lots of people talking to each other on various topics including those with their own questions. The messages just come in a stream, much like you'd expect of something designed around chat but like, if I get up to make coffee and miss someone's reply to me, how would I ever find it. Or conversely if my question is not immediately answered but someone joins the room later that could have answered it, how would they see it?
If I make a post here on Lemmy, it's open and around for anyone to answer it for some time. Theoretically it's around forever but in reality it's more like however long it shows up on people's feeds but either way it'll be longer than a few minutes or seconds.
Matrix is for chatting, not posts.
When it goes well you get live, interactive support and get your question answered fairly quickly. Nice and convenient. But as you've said already, it has drawbacks and it's where forums and things like Lemmy come in, where sometimes you can get replies days later.
They're different systems that reach different audiences. You use whichever based on the needs and complexity. What sucks is when the chat rooms develop some knowledge that doesn't get known outside and it's also not indexed anywhere on the web. Some things are better discussed in forum format (or mailing lists if you're very oldschool), while others are just better interactively and the back and forth on a public forum would just be painful.
Usually there's a bit of an overlap at least, where users are usually in Discord/Matrix/IRC and some forum or reddit or fediverse community at the same time.
This is what I find so odd about modern messaging systems being used in the manner that they're used. I get that the immediacy of conversation is sometimes extremely helpful for discussing topics and I can understand why like minded people would then want to hang out together to have those conversations, but like, it's also kind of flawed for this because of the ephemeral nature of conversation. That's why I wondered if this flaw had been addressed through some forum-like features.