It seems that concurrency issues are not addressed in this article because PEAR Cache design did not made them mandatory.
This means that you may have your cache files trashed by simultaneous accessses on which more than one attempts to update the cache contents. This may make PEAR Cache to file useless as it can't assure cache data integrity.
PEAR Cache also can send the cached data to a database but this is just an academic type of container because in real world database access is much slower for this purpose than using local file system files. Usually you local file system based caches to avoid database accesses.
Another point is that PEAR Cache db container uses SQL code specific to MySQL, despite PEAR DB can interface with other databases.