I'm not knocking hashtags, it's good to use them too! :)
Here's what I meant by "more powerful":
If you post to a group, everyone who subscribes to the group will see your post. If you use a hashtag, only some of the people who follow the tag will see your post.
Hashtags are filters for your server's Federated timeline. If a post isn't on Federated already, it won't be visible in hashtag searches or subscriptions.
Group posts are actively pushed to all servers of all its members.