Commits and merges on SVN subdirectories considered harmful?

We have several large subprojects inside our main SVN project root.
I commit and merge only my subproject when working with our release branches, mainly because it's faster.

However, a colleague pointed out this reference to merging subdirectories in Version Control with Subversion (a.k.a "The SVN Book"):

• For long-lived release branches (as described in the section called “Common Branching Patterns”), perform merges only on the root of the branch, not on subdirectories.

Unfortunately, this is the extent of the warning. The linked section does not give an explanation either.

Is committing and merging SVN subdirectories harmful for release branches?
What about short-lived feature branches?