Resource blocking leading to resource starvation
Lock contention limits scalability and adds complexity.
Priority inversion. High priority threads/processes cannot proceed, if a low priority thread/process is holding the common lock.
Balances between lock overhead and contention can be unique to given problem domains (applications) as well as sensitive to design, implementation, and even low-level system architectural changes. These balances may change over the life cycle of any given application/implementation and may entail tremendous changes to update (re-balance).