tl;dr:The costs incurred of hiring a junior engineer isn't always realized. "If a team isn’t willing to invest at least 1–2 years, they shouldn’t be hiring junior engineers." Investment comes in mentorship, manager's loss of productivity, and other ways outlined here.
tl;dr:On behalf of post-commit reviews, where comments are addressed in follow up PRs. Developers can iterate faster, pull requests are kept small, code reviewer can batch review, along with cons and considerations.