Conclusion: The search function is nearly useless.
Could you give a real world example of a search phrase you've tried and received useless results from?
To put all this into perspective, IP Board uses PHP and MySQL. For searches it's highly likely standard SQL is being used against text fields. Put another way, that means the search is extremely limited. Standard SQL full text searches are either like fishing for trout with dynamite, or very precise, but mind numbingly slow.
If there's a bug in the SQL query, '+term word phrase' could return all topics with a single instance of term, word or phrase in them. If the search is fine, all the topics returned would definitely have term in them, and may or may not have word and/or phrase in them.
I have to develop MySQL text searches all the time for various applications. If a few of the people who've had headaches searching submit a few real world search terms, I can probably work out where the problems lie and suggest solutions. If not solutions, then fairly precise reporting why they're probably failing, which could be useful to the developers.