Do ct scans with no contrast always miss tumors in the cerebellum?

Not always: Brain CT scans without contrast do not always miss cerebellar tumors. A tumor whose density is different from that of its adjacent brain tissue, or whose size is such that it pushes away adjacent brain tissue (distorting the normal shapes and outlines) will likely be visible even on noncontrast scans.

No, they probably: can visualize most of them, although often not very well , and not with the degree of sensitivity that a contrast enhanced scan has. That said, MRI is much better than CT for evaluating cerebellar/posterior fossa lesions.