Certain risks of infection maybe but I have done it before, froze some mountain spring water, same water I used in boil as to make sure there was no added taste from the whatever water ice bags are made with.
My friend did it and said it cooled the wort real good. I think he used bagged ice. I didn't want to tell him that it may have been a bad idea due to plastic leeching from the bah and possible chlorine. If the ice is definitely " clean", is there any harm in using it as opposed to using water?
Best idea would be to boil all 6 gallons, but not everyone can do that, in this case if you feel like you must use ice in the wort, which is. It necessary, ice bath or immerison cooler is just fine, I would say make sure it is clean like you say, and maybe also equally importantly make sure it tastes clean or has no taste at all, some ice taste disgusting to me, gives whatever I put it in a nasty off flavor.
You can just freeze the jugs of spring water. That way you don't have to deal with the worries of ice trays. I put 3 chilled jugs of spring water into the deep freeze when I start my brew process. When it comes time to chill the wort, I sanitize the outside of the jugs and cut the tops off. quick cool and it's sanitary. All my brews have come out great thus far. Clear beer, no signs of infection.