Plenty of sites have terrible search and discovery stories. I routinely search in a general purpose search engine and filter to the site whose results I want because it's faster and more reliable than trying to navigate the site itself.
It's been a decade since the majority of sites had a usable search of their own. Many that still do are just hosting a site-specific Google search at this point.
I think you’re implying that Reddit’s search is subpar, which I agree, although it seems it has improved slightly, recently. When I want firsthand perspective on something I usually append “reddit” to my google searches instead of searching reddit itself, even if I have a reddit tab open already.
On relatively sparse, self-post (fow or no link) subreddits, Reddit search is fairly useful for searchin posts only.
Comments are not indexed.
At volume and with links, search breaks down from both insufficient context (esppecially with editorialised, sensationalised, or vague titles) and excess low-yield content.
Better yet, if I already know I want a particular doc or site, I'll go directly to that site instead of hopping through a search engine first.