I freeze-concentrated some agave mead a couple years ago. I did the whole milk jug in the freezer routine. It worked ok. It's not like you are going to go from single digit ABV to 20-30% unless you have access to freezers that can go well below 0F.
I did several passes, starting with a gallon and got down to about a pint. At about a half gallon I noticed from tasting the ice that I was losing some alcohol that was binding to the water trapped in the ice. After that I kept going just to see what happened but there was definitely a point where I wasn't getting any more alcohol concentration, just flavor concentration. Probably losing some alcohol, too. To get more concentration you really need lower temperatures rather than multiple passes. Most of the websites about the process explain the concentration to temperature correlation.
The end product wasn't terrible but wasn't great either. It started out around 8% and just by guessing from drinking it I probably got it in the low teens, maybe the mid teens. The biggest problem, if you really try to concentrate it more than once or twice, is all the dead yeast and all the minerals from the water concentrate and become a strong part of the flavor profile. For me the saltiness wasn't too bad because it ended up tasting like syrupy mezcal (I seem to think I just used tap water in the mead) but if I were to try any freeze concentrating in the future I would use very low mineral water and lager first to try to drop out as much yeast as possible before trying to concentrate.
If I had the freezer space again I'd probably try freeze concentrating something else but I would be a lot more precise about the process.