You are right. No visa does guarantee you entry. But it makes trouble less likely than on a visa exemption - espcially if you have an extensive border crossing history.
No, I am not wrong about this. Land borders crossings are limited to two per calendar year on visa exemption entries. There is no (theoretical) limit on any border crossings with a valid visa. Of course every border crossing (visa or not), is still at the discression of the immigration officer. Not sure what happened in your case.