For searching instances and communities, you can use lemmyverse.net. It's a bit obnoxious to have an external service for that, but they have done a good job of filling a hole in community search, I think.
For comment search...yeah.
Reddit was infamous for having a useless comment search engine for many years, until people just started doing site:reddit.com searches with Google. Like, Google partnering with Reddit had a site famous for great content and terrible searching of it (to the point that one used Google to get by) meeting up with a search engine that had great search but indexed a lot of garbage (to the point that one would specifically do site searches on Reddit to get useful information). Even aside from AI training, I can see why they partnered, my own apprehensions about the anti-competitive aspect aside.
Google doesn't, as far as I know, have a good way to search all Threadiverse sites.
Kagi specifically indexes the Threadiverse, has a search lens for it, and can assign something like !tv or similar to do so.
I don't know what the status on other search engines is. Might be that some other engine has since added support.
There's no native full-text search in the UI (and an individual instance doesn't even see all of the comments made, so it cannot index them for full-text search).
Hmmmmm.......I don't REMEMBER writing this post.....or having that account......wait, are you a different person with my same opinions??? Ok, quick, what are your opinions on POGs???