I’d also like to point out that the entire Internet is a third party you have no control over which you open your external traffic to everyday.
Not really. Proper TLS enables relatively secure E2E encryption, not perfect, but pretty good. Adding Zscaler means, that my entire outgoing traffic runs over one point. So one single incident in one single provider basically opens up all of my communication. And given that so many large orgs are customers of ZScaler, this company pretty much has a target on its back.
Additionally: I’m in Germany. My Company does a lot of contracting and communication with local, state and federal entities, a large part of that is not super secret, but definitely not public either. And now suddenly an Amercian company, that is legally required to hand over all data to NSA, CIA, FBI, etc. has access to (again) all of my external communication. That’s a disaster. And quite possibly pretty illegal.