Assuming the mother is a normal, loving, capable, caring one...
For the mother who has a husband who isn't involved with helping care for the kids, I'd say it's just the amount of day-to-day work that can be the biggest challenge.
For the mother who has a husband who does get involved but is clueless about what's right for children, child development, etc., then the biggest challenge can be having someone who works against her to the point where she either goes along with what's really not best for the children, or else that she's got (essentially) a live-in enemy with regard to what is best for the children.
Either way, lose/lose. Well, more accurately (and depending on how many children there are), lose, lose, lose, lose, etc. etc.