My understanding is that search on the microblogging side of the fedi is intended to be “broken” (from the view of someone expecting a Twitter-style search); hashtags are for opting-in to global discoverability whilst without them your posts are intended to be stumbled upon and/or passed around rather than sought out.

Well it's a bit more complicated. A really significant reason search isn't that comprehensive even on a big instance like mastodon.social is that Mastodon prioritizes privacy and has made it optional to be included in the search results with mastodon.social also opting to make it disabled by default when they added it.

A second problem is that if you're on a smaller instance you may not be seeing enough posts because they don't propagate there. This also affects hashtags. There's projects like Holos Discover fediverse search engine and Fediscovery that are addressing this problem but they won't change the fact that many users simply have indexing their posts for full text search disabled.