I’ve had problems with NFS on Linux basically forever, and it became significantly better AND worse under NFSv4. It got better in that locking mostly works under version 4.2, so things like alpine work correctly where the mail spool is NFS mounted. It got worse in that sometimes mounts fail to mount, especially if a server goes away and returns.
I noticed recently that this is a much larger problem with servers that have a lot of entries in /etc/exports. This lead me to investigate whether there might be a limit on the file size. Well, I did not find a limit but I did find that the sanctioned method of exporting the same file system to multiple hosts was to put all the hosts on the same line separated by a space and each host followed up by options, where I had had each host on a separate line.
So I changed the format of /etc/exports from the old form which is how SunOS expects it to be formatted one, to the form officially sanctioned by Linux and will see if that helps.