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.
Damn the devs should do something about that. Lemmy's search seems alright so far.
You're able to see tagged posts from people on other instances when browsing hashtags. Unless I'm misunderstanding what you mean
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.
Ohhhhhhhh. That...really sucks actually. That might be why I'm struggling to find anything on certain topics I'm interested.
This seems like a huge issue but at the same time I understand that instances have limited resources so I don't really know how to solve it...
Very few instances have full text search enabled at the moment, usually using a special version of Mastodon. Iirc it will be coming to standard Mastodon (as an option) in the future.
Decentralised and open source social network.