My son was in a nanny share the first 2 years of his life (starting around 4 months on). We loved it & I recommend it to anyone willing to listen!!
There are definitely trade-off but I think there are ways to plan for them.
First, one thing you get with a nanny share vs daycare (possibly - depends on the kinds of daycare options available to you) is more flexibility & control over childrearing philosophies or approaches. We selected our nanny based on her willingness to go with our AP parenting - she wore the two babies (one at a time!), took time to help our son fall asleep (he wasn't a great sleeper, whereas the other baby was), dealt with cloth diapers, gave my son expressed breast milk & later fed them nutritious snacks & meals, no TV. In our area, we couldn't have gotten that at a daycare.
As you recognize, though, stability is a big selling point for the daycare option. I admit that it was sometimes very stressful when our nanny was sick. In our second year we formulated a pretty solid plan for dealing with this - we pooled our resources & shared care with the other family. So instead of having to miss a full day of work or to pay someone new a full day, we's swap. If the other available parent had an important meeting, someone would cover them & watch both kids, then we'd switch. It wasn't a perfect plan but it worked.
I also disagree with PP about the germs. DS swapped germs with his playmate & at the park etc. He was in no way at an immunological disadvantage when he started preschool. He's in his second year of preschool & has run a fever once in that whole time! I don't think daycare is an advantage on that point.
Also, I disagree about safety. I think there are safeguards available for day area that are positive & above & beyond what is available in a home setting. But in my experience, these things aren't a concern if you find the right nanny for your families. And with a share you have four people, all with parental instincts, giving input on who to hire. Hiring took a long time because we all had veto power & used it but it was worth it in the end because I never once wished I had a nanny cam or any such thing.
There are always risks in the world (in a daycare, with a nanny, with a parent -- my son ended up in the ER once on my watch & never a scratch with the nanny...). You need to make a decision based on what will work & feel right for you & your family.
If we have another, we'll definitely be trying to create another nanny share!
Edited to add: When my son was 3 we hired a part-time babysitter to care for just him. I wished I had a nanny cam because something just didn't feel right. We fired the babysitter - I figured, if I felt that way then something WASN'T right. I'm a big believer in going with your gut in child care decisions - a child's happiness & well being & a parent's peace of mind (connected, as they are) is just so important. So whether it's a daycare or a nanny share - if it doesn't feel right, keep looking!!