Okay, there's a little concern here about "bad alcohols" in freeze-concentrating. Here's my 2 cents' worth:
1. First of all, freeze-concentrating will get you a mead (or wine, beer) with an alcohol content similar to fortified wine, like port or sherry. It removes a lot of the water and (mostly) retains the alcohol, but it also retains some of the other stuff, like sugars, etc.; that might be left behind. I tried it and it is hard to get pure ice out without a little of the liquor leaving with it. I put the ice in a colander and allowed the liquor to drip back in (in a frozen environment), but a little stayed behind. I didn't measure the ABV, but I suspect it might have been in the 20%+ range.
2. There is a concern about an increase in "bad alcohols" (esters), like methyl, dimethyl (as opposed to ethyl) alcohol as a result of freeze-concentrating. Did you know that, in some beverages these esters are intentionally left behind? Distillers of whiskey and some brewers of lager take them out, but vodka makers and, I think, English ale brewers leave them in. They do it because the taste of their respective beverages requires them. Yes, methyl and dimethyl alcohol can be toxic, but so is ethyl alcohol.