It's not FOSS (neither client nor server), you can't control any of the infrastructure (and therefore guarantee any level of privacy), sovereignty of the data,

It's not FOSS (neither client nor server), you can't control any of the infrastructure (and therefore guarantee any level of privacy), sovereignty of the data, the data itself, and policies for use which may not align with your own. All before you get into the technical differences in applications and protocols - good and bad.