I've found reddit first party search to be pretty lack luster. More often than not, I'd have to fallback to Googling "site:reddit.com/r/subreddit-of-interest <terms>" and while better, that's about a 50/50 a lot of the time.

Reddit does this thing where it advertises other threads on the same page of whatever you're viewing. So sometimes the promoted content was the thing actually relevant to the search and not the current thread. And this would be fine except it's not static/deterministic. The promoted content usually changes between when it's indexed and when it's accessed at a later time - so the thing that made it relevant to the search often isn't there anymore. So you have to either view the cached paged if available, go back and try different results, or refine your search.