Whilst Samsung sells the DeX dock for the Galaxy S8 it is rather expensive and overkill for most users. With Miracast support onboard you're probably thinking you're covered for quick slideshows of photos or to pop a video up onto a large screen TV.
It's technically possible, however I can tell you it's going to be a disappointing experience. The extended screen ratio of the S8 - and this is also true of the LG G6 - means that your mirrored display will suffer from extreme letterboxing when shown on a normal TV screen with 4K or 1080p resolutions.
It isn't a total deal breaker - you could use Chromecast to get media onto a big display, but whereas Miracast is in many (if not most) new TVs, using Chromecast means carrying an adapter around and hoping your destination display has a free HDMI port.
Incidentally, for those looking to work with a DisplayLink dock the same issue with display letterboxing is present. So without the DeX dock there'll be no impromptu 'smartphone as a laptop' sessions here.
Minimal use case admittedly, but worth knowing if you're considering any smartphone with an extended display ratio.