Is that because you loose ground?
No. It’s because you now have two live conductors to deal with and domestic equipment safety schemes are designed with one in mind.
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Ta. I think…
I can’t really answer the mainsy stuff without getting caught-up in vested interests, but second what Pete says. So-called balanced mains is a minefield vying almost with people who put copper bars in place of fuses in their kit . . . . . . 
Big toroidal transformers saturate with hilarious ease, and often magnetostrict loudly while so-doing, so you need to ensure there’s no DC offset upstream.
Almost everything people routinely do to mains supplies adds as many problems as it removes, usually by restricting current headroom, or nailing-on dangerous bullshit and technobabble and raping your wallet for the privilege - the flipside being that the very real benefit of getting it right gets dismissed as “foo”.