Hashtags do not federate. Sure, they federate, tagging along to posts that are sent to servers of followers. And though you can't subscribe to hashtags, you can subscribe to hashtags, once they happen to have arrived at your server.

But the important use of a hashtag, to create an informal space around a subject you can explore and subscribe to, doesn't work in the fediverse. If I'm interested in "# AmazingSubject" and subscribed to it, and you post your amazing take about it with "# AmazingSubject", I won't receive it. Not unless some specific conditions apply like we're on the same server, or someone on my server follows you, or someone with the right connections reshared your post, or our servers are on the same relay.

But if you need the info about e.g. flood warnings, track a news topic in your job as journalist or security researcher, support customers tagging your product, or want the latest information about your hobby, this is not good enough.