what’s the impact of using Zscaler proxy services?

https://www.zscaler.com/

Is it true,

• that they play “man in the middle”
• and use false certificates,
• and decrypt your encrypted Internet communication?
• …

I like, that they let non-standard ports through as well, e.g. https://diskstation.home.com:5001/webman/, that way I can access my home office NAS from a customer’s Intranet.