Performance and Scale out. Synology or QNAP can't handle an enterprise demand, and just doesn't grow when to an enterprise solution such as an Isilon install. I am currently maintaining 2 500TB isilon clusters.. If I fill them up, I just add additional nodes. Additional nodes = disk space, and compute power. You don't get any of that with a Synology or QNAP device. There is something to be said with scale out vs scale up.
BRB, will be maintaining HUGE enterprise stuff that I'll stealth brag about in a consumer NAS thread later.