Problem of mastodon is that you are only able to search hashtags, and only on your instance. So if you are on small instances where nobody is talking about topics which interest you or are not using hashtags, you are not able to find anything, because fulltext search is not supported.
enkydu
joined 1 year ago
I just subscribe to communities on multiple instances, so I can see all posts on my home "subscribed" page
You can see tagged posts only if they are known to your instance aka somebody is following account, which used mentioned tag, but search is not able to really search all posts with hashtag in whole fediverse. So if there is some niche instance about some topic, you will never find posts from there, if it was not federated by somebody following accounts from this server.