Especially in comparison to fixed opening hours of a day care center, an au pair offers unmatched flexibility in distributing the child care times over the day and week.
But the family has to bear in mind, that (depending on country – the exact figures for each country can be found in the country information) an au pair is only allowed to work between 20 – 30 hours per week (only in the USA it’s up to 45 hours) so it’s definitely no full time child care option.
Additionally the au pair needs to have the opportunity to participate in language classes which might interfere with the times when the family actually needs the au pair to look after the kids.
An au pair provides live-in care, which e.g. means that in case of emergency the au pair will normally be available to immediately look after the kids.
But of course this also means that the family needs to have a spare room which can be used as the au pair’s room (and if this was the guest room before, the family might not be able to accommodate guests anymore…)
Au pairs normally don’t have professional child care experience, they have usually just done some babysitting or have younger brothers and sisters.
So the family will have to spend some time to get the au pair acquainted to all the necessary routines and also their discipline methods.
The family might see this as a disadvantage, as they can’t expect professional knowledge and advice – or as advantage, as the au pair will normally fully support their parenting style, feeding habits,…
That's an interesting advantage especially for babies: As their immune system isn't working at full capacity yet, being in contact with other (ill) kids (as it is the case especially in day care), will lead to frequent illness meaning that the baby will stay at home and need to be looked after at home - and often it also passes on the illness to the whole family.
As au pairs are only allowed to stay for a certain time span (in most countries one year), kids have to get used to comperatively frequent change of their care giver / attachment figure. If this is positive or negative to the development of the child very much depends on the character of the child and also how close the bonding with the au pair is.