How does ionice work with multiple drives?

I understand how ionice can help you when you have multiple processes requesting access to the same disk resources, but how does it work when you have multiple disks? For example, you have one rsync operation moving data from *Drive A -> Drive B*, and another rsync moving data from *Drive C -> Drive D*. In theory, since they are not competing for resources, ionice'ing one of these rsync processes shouldn't change its throughput. Is this how it works, or will it still impact performance? Additionally, is there some "upper limit" on total *I/O* one might experience on a *linux* system that is independent of drive speed? Like if you hooked up 100 *SSD* drives, at some point would the *OS* run into a bottleneck aside from drive speed?