I read that the vSphere Client (HTML5) doesn’t have the same features as the Flash client or the C# client. Is that true?
The vSphere Client has the same features as the Flash client, since version x. Some things have been implemented differently, but they are there.
Check out the list with functionality updates for the vSphere Client at https://docs.vmware.com/en/VMware-vSphere/6.5/rn/vsphere-client-65-html5-functionality-support.html