The only issue is the obvious one that the manufacturers are usually
fairly vague about the exact usable size of each disk.  If you
bought half the disks from one manufacturer and half from another
then you should be able to pair them up and use the minimum of the
size of each pair.

In a similar vein, if the drives have different speeds and you may be
wasting some performance. The faster drives may be forced to
sit around waiting for an I/O operation to complete on the slowpoke.

Jeff