For UCD, both Biology and Bioengineering are competitive and popular majors. Declaring a major depends upon how the applicants are admitted into each university.
For example, UCSD admits into the university first and then into the major, so it is possible to be accepted but as undeclared.
For UCD:
Admission decisions are made based upon the qualifications of the applicant pool and the number of available spaces within each academic area:
College of Agricultural and Environmental Sciences admits by college
College of Biological Sciences admits by college
College of Letters and Science admits by division within the college
(Humanities, Arts and Cultural Studies; Mathematics and Physical Sciences; Social Sciences)
College of Engineering admits by academic department
Students applying as “undeclared” or “undeclared/exploratory” are considered within the college/division to which they applied.
Undeclared should not have a negative impact.however there are many majors (STEM) that are very difficult to switch into later so the best approach is apply to the major of choice.