The limitations of remixing a mono track to 5.1 simply stem from the fact that one channel has to provide the information for 5 channels and the sub. Compared to even a stereo track expanded to 5.1, this mono to surround remix will tend to sound thin and quite artificial.
Very often, new foley and fx work is introduced to fill the channels, and can lead to a fantastic, strong sounding remix (eg, The Exorcist TVYNS or The Godfather HD Restoration), but the problem here is that the film's original sound is partly being scraped for the sake of creating a modern, immersive audio experience. Many people, myself included, like to enjoy the work of the original sound mixers of the time, therefore only really appreciating subtle remastering tweaks used to polish the track. In my opinion, the best surround remixes stay faithful to the source and do not ake on a thin sound, ie. just the music is expanded to the surround channels and the rest of the audio remains close to the centre of the mix, with some added bass to the sub for good measure.
That's not to say that a mono/stereo track can't sound fantastic as PCM audio: it's lossless, and as previously mentioned, can be rendered at extremely high sampling and bit rates. Arrow did a great thing by including the PCM 2.0 audio on the Dawn Of The Dead bd. It's common enough practise to just use an older, heavily compressed Dolby Digital 2.0 track, but this is thankfully starting to change.
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