In my mind the house can be kept clean via a sensible directory structure.
If I separate the drives into multiple raid arrays each array will have a space limitation and each one will need to be managed.  If I have only 1 large array, I can add a disk whenever anything (logs, data, etc.) needs space.  So, its easier and more efficient from a space management perspective to have only 1 array.
However, if there are performance advantages to having multiple arrays that could win the day, i.e. moving toward multiple physical arrays, like we would have done with spinning disks.  I just don't know how performance is affected by creating multiple raid arrays versus 1 raid array.  I would guess it works similar with spinning drives but maybe not because of the speed of the SSDs.