I have found NFS to be unstable. I am running unraid within ESXi. I was originally mounting from fstab, but ended up with a lot of "stale NFS" errors. Form another post on this forum I started using autofs. It seems to have marginally improved, but I still end up with errors telling me a directory doesn't exist, which I believe to autofs for "stale NFS" errors.
Just some background, I'm running ESXi 5.5, with NFS user shares, X9SCM, and an IBM M1015 card.