I am not saying DS1 is an angel, but he's a fairly normal 13 year old with slightly better then average manners and reasonably domesticated as well. He gets asked back to other people's houses, exchange partners and so on, so I am not putting rose tinted glasses on here.
The reasons the nannies left were very different.
First one strung us along all summer by pretending she wanted to work here in order to negotiate a higher salary from another family (I think), and then failed to turn up at the last moment, leaving us in the lurch completely, moments before my return from maternity leave, and consequently she was blacklisted by Tinies.
Second one (recent graduate from Famous Training College) was incredibly immature and more or less pulled out by the college after various things happened, including several tantrums by her, minor neglect of kids and two AWOL incidents, as well as crashing the car. Wierd year group, Frak has some hair raising tales to tell about other girls from that batch as well. 
(Any more gossip, Frak?)
Third one (also from Famous Training College) stayed one hour and then decided she was too tired to work, having done 10 days' work over Christmas. Just disappeared off claiming her mother had had a heart attack (which later proved to be more or less untrue). We were then offered her by Eden so it seems while she was still contracted to work for us she had left and registered with them at the same time without telling them what was going on. All very odd.
Two very experienced temps fine, reassured us we were normal and not sociopathic and the type of people to scare nannies off.
Subsequently latest nanny, who is older but who has worked in nurseries for most of her career before starting work for me. Some personal issues which are impacting on her work.
So it's not us, honest.
I have had au pairs before, by the way, but we worked out only 10% of ours have really been OK and I am not sure I can handle the disasters any more, plus there's a toddler involved now (hence the nannies).