Cornrows and braids are not haram at all for Muslims in general, but customs vary in some sects. However, there are different rules for men and women in this.
Women do not have to undo their cornrows for wudu or ghusl, but proper dry ablution must still be done for wudu. For ghusl, a woman must be sure that water wets all of the roots of her hair.
Men on the other hand, must undo their braids for ghusl. This means after every marital relation with their wives or at least every Friday during the ghusl before Jumuah. So simpler styles are recommended for men who wish to wear braids.
I’ve been told by a local African Muslim woman in Israel, that women in her sect are required to undo their braids during menstruation. Braids have nothing to do with observance or general ritual purity though. So ask your local Muslim women whether or not one is okay to pray in braids at their mosque.