The problem with Guppe is that it automatically federates all the post content unlike with hashtags which are just a marker. If someone spams in a hashtag n one server it’s going to suck but unless people follow them it’ll be limited to that server for the most part. With Guppe groups if they spam to them it gets boosted to all servers. It’s not just a tag that gets cluttered, it forces the content into all the other instances by way of boosting.
Even with the Hashtag misuse that’s not as bad as group spam, since people spamming groups can reach further than they otherwise would just spamming on their server. Which is why being able to moderate groups is important. Also with Guppe’s system you can create any group you want without authorization, that’s a problem because malicious people can create groups that aren’t acceptable including ones with racist and illegal content and use it to distribute that across the network. I found a few of these without much digging and I quickly reported them to Mastodon.social and got them suspended but they’re still there on the other servers.