There are many kinds of premises, in every possible field. I'll limit this question to metaphysics, although it can definitely be applied to each and every scientific/philosophical study.
For example, there are assumptions like (a) "all male are mortal", and assumptions like (b) "God exists". While we might (rationally) suggest that (a) is "more rational to assume" than (b), what makes us say such statement? How would we conclude that a certain assumption is more "rational"/"less powerful" than another assumption? On what "scale" would we determine such conclusion? Occam's razor basically says that less assumptions is ultimately more rational, but what if we have one "powerful" assumption against many "small" assumptions?