I'm not sure this is the case. Like sure, slack / teams can see all your data, but if you have a trivial problem such as being in the EU, you suddenly now have

I'm not sure this is the case. Like sure, slack / teams can see all your data, but if you have a trivial problem such as being in the EU, you suddenly now have a compliance problem if specific data gets put on the service. Run mattermost on your own server and many of the problems go away. Mastodon is a bit different because it's federated, but I assume since it's more akin to a social network, it's similar to running a community support website compliance wise.