Can I just give you my personal experience with regard to this? My nanny (4 days a week, but shorter days than usual) did not have her own DC, and I used to work part time from home so was around quite a lot. She came straight from a nursery, hadn't finished her NVQ3 (and still hasn't!) and had no previous nanny experience.
After about 10 months in the position she started to get 'opinionated'. She began to comment on various elements of my parenting and children's behaviour. The comments started from small, and grew larger and larger. At first I felt quite defensive and was then quite annoyed. I sat down with her and spoke to her about it a few times, but things just kept getting worse. Annoyance gradually moved on to absolute fury. In her mind though, she was the 'expert' and I suspect she felt she knew my DC better than I did.
In the end it got too hard and I cut her down to 1 day a week, and found a CM for 2 days. But the resentfulness on both sides stayed and after a massive blow up I had to dismiss her on the spot, and paid her in lieu of the notice period.
My nanny also had a LOT of days off sick, which she ended up using her AL for in order to not be without pay. It was a pattern, and I understand that she has continued this with her new employer - who is also ending her employment contract soon. These patterns don't change without a rude, sharp, shock. For many people, only dismissal will actually make them realise just how unacceptable this is.
If these comments are not stopped NOW then I am concerned that it will just get worse.
Quite honestly, the first time she made a comment like that you should have put her back in her place - she is the NANNY, and not your HV, parent, family member, friend, whatever else she might think she is. As a nanny she is there to assist you, make suggestions and that's it, she is NOT there to JUDGE you!!!!!! If she wants to think it, fine, but she can keep it to herself. (I say this but it's easier said than done, I know. If it had been that easy I would never have ended up in the situation I did!!!)
I'm not saying change to a CM, but I myself didn't want someone back in my home after this experience, and wanted some time to re-evaluate before I think about hiring a nanny again.
Also since this dismissal in early December, my DS1, who is 3 1/2, has barely mentioned her. He doesn't seem to miss her at all, and is very happy at the CM, making the transition so smoothly I was astounded!!! DS2, 17 months, is just as happy, and adores being at the CM.