Having used both, if you can somehow navigate a phbb board then you can easily navigate discord. The only thing stopping you is you.
Maybe navigating is the wrong term. It’s just impossible to find stuff relevant to me on discord. On any given larger server, there may be a few channels I could be interested in - but they are just a single chat log, often with lots of off-topic spam, and many different people having almost separate discussions at the same time. On any given larger phpBB, stuff is mostly separated into different threads with all the off-topic posts being delegated to a single thread. It’s better searchable and better organized.
What even is “relevancy”? Their search is just a search by matching keywords. There isn’t a magic algorithm discord uses. Every time I had an issue with some sort of bug or function I just search for specific keywords and 9/10 times I find something. On the odd-chance I don’t then I’ll behave like a human being and ask. I just don’t get what’s wrong with that? You can already limit those keyword searches with specific constraints so you don’t get much noise.