Technically, the term “impacted major” is used by the CSU system – not the UC system. In the CSU system, your chances of admission are indeed very good for non-impacted majors, since they should have room for all qualified applicants. As long as you meet the minimum criteria, admission to non-impacted CSU programs should be guaranteed.
The catch is that many of the popular majors at many of the popular CSU campuses are impacted. In that case, your qualifications will probably need to exceed the minimum criteria in order to get in. Summary of impacted CSU programs here:
https://www.calstate.edu/sas/documents/impactedprogramsmatrix.pdf
The UC system uses the terms “capped major” (UCB/UCSD) or “selective major” (other UC campuses) for the programs in highest demand. The link in Post #1 above will give you an idea as to the transfer acceptance rate for specific programs at specific UC campuses.