I am using:
Galaxy Tab S 10.1 (SM-T800)
TMobile S5: Stock rooted late May build, Xposed is installed, Knox apps are disabled (about all the possible relevant things I can think of)
So I fire up Sidesync 3.0 on both devices to try out this much touted feature. I get connected just fine, but I cannot operate the phone's touchscreen at all from the tablet. There's just no response. However, the menu/home/back virtual buttons do work.
A search has turned up empty, which included trying one of the level one script readers from Samsung support. They literally had no information about tablet to s5 connections.
Disabled my exchange administrator thinking it's some security lockdown on my phone, but nope... removed my account, rebooted, didn't fix it. I'm out of ideas.
I figure this is the place to ask. I'm figuring I'm right.