I am not aware of any issues, you just wont have the benefits of using Datastore Cluster features such as SDRS. We have multiple Datastore Clusters - for different disk type/speed (10K FC, 15K FC, 10K SAS etc).
Also consider if you have a VMFS Datastore Cluster and then introduce an NFS Datastore - this would have to be a 'stand-alone' Datastore as a mixed cluster is not supported (of course if you had 2 or more you could make an NFS Datastore Cluster)