With suEXEC PHP, each PHP process has its own cache memory, it wastes a lot of memory if many PHP process running. It is true for all opcode caches.
eAcc store cache on disk, we thought it is a good idea to put the disk cache under /dev/shm, make it effectively an in memory cache shared by all PHP processes, however, eAcc causes high load for some users, maybe due to bug with certain PHP release.
So, we do not have a perfect opcode cache solution working really well for shared hosting. If the bug in eAcc has been fixed, it will get our recommendation.