What is your technique for estimating in the case that the tech stack is (at least in part) unfamiliar? Can this technique be applied by junior developers with only a few years experience?
Few years of experience should be quite sufficient, although I think overall team culture is probably more important. I've usually done this type of estimation together with teams where there's already good communication, but when that's not the case something like pointing poker to open up safe discussion and uncover uncertainties may make sense as well.