Training a custom chatbot is easy. Having it reliably retrieve the data you want is hard. For any given question, you can get it to look at a maximum of 4k tokens (16k characters, or 3k or so words). So when you want your chatbot to have access to a LOT of data, you usually end up having to work around that - typically by:
1. chunking up your documents into ~2k word chunks,
2. and then getting the query, doing a search using that query, and getting the top N results back,
3. running the prompt on each result, and then
4. gradually combining the results of each run of the prompt.
Each step is a little lossy, and there are quite a few steps there, so the bigger the corpus gets, the lossier/less useful your bot is.
Now that being said, there are ways to improve that, but for now at least, its unlikely that a single slackbot is going to solve all your knowledge management problems for you.