I know someone else has mentioned it (far back in the last thread but also like probably only 4 hours ago!) but I just want to reiterate... they are so new that they are likely to not have had their first proper argument yet. They've not gone through any real hardships (other than perhaps Carrie's woe at her previous break up, which like, yikes).
Does she know how they repair their relationship after conflict?
Does she know how he'll respond if she goes through a serious event (like grief, or illness of her or a loved one, etc)? Or what about if he goes through a hardship like that, does she know how he will be? These are the things that test a relationship and this relationship just hasn't had any tests yet.
You can't predict everything and perhaps I'm on the more cautious side but I could never get engaged to be married to someone without knowing these things. I'm Carrie's age, went through a breakup of a 6 year relationship just before she did (though in fairness I ultimately made that decision) and I am 3 months in to a new relationship and I just can't imagine the idea of being engaged in a couple of months time knowing how little I know about my new love. My god.