The exact quote from the ext4 Wikipedia entry is
However, Red Hat recommends using XFS instead of ext4 for volumes larger than 100 TB.
The ext4 howto mentions that
The code to create file systems bigger than 16 TiB is, at the time of writing this article, not in any stable release of e2fsprogs. It will be in future releases.
which would be one reason to avoid file systems larger than 16 TiB, but that note is outdated:
e2fsprogs since version 1.42 (November 2011) is quite capable of creating and processing file systems larger than 16 TiB.
mke2fs uses the
huge types for such systems (actually,
big between 4 and 16 TiB,
huge beyond); these increase the inode ratio so that fewer inodes are provisioned.
Returning to the Red Hat recommendation, as of RHEL 7.3, XFS is the default file system, supported up to 500 TiB, and ext4 is only supported up to 50 TiB. I think this is contractual rather than technical, although the Storage Administration Guide phrases the limits in a technical manner (without going into much detail). I imagine there are technical or performance reasons for the 50 TiB limit...
e2fsprogs release notes do give one reason to avoid file systems larger than 16 TiB: apparently, the
resize_inode feature has to be disabled on file systems larger than this.