It may depend on the specific brand and model of the Roku television, as they are made by a couple of different brands.
I have one of the very inexpensive ones in our guestroom. Alexa control works very well with it except that it can’t turn it on. 
that’s not uncommon with smart televisions.
So we added a Switchbot IR blaster, less than $50, just to be able to turn it on with voice. Switchbot also has a manufacturer-provided SmartThings Integration which works quite well.
I wrote a thread about it last year:
Switchbot - Simple “power on” for some TVs (IR blaster)
That said, if by “sync” you mean you want to be able to see the correct status in smartthings if someone turns the television on with the button remote or by voice, that’s a different issue. We didn’t bother with that, but I do know some people have the television plugged into an energy monitoring smart plug so they can see whether it’s on that way.