Whatever pros you can state for building Ravenous Hydra on Nasus, I can state the single con of him missing out on q stacks, and it negates any pro. Unless you are building AP Nasus (Which does his dmg off his e and doesn't stack his q) and even in that case where stacks don't matter, neither would the wasted stats of the Hydra on an AP Build. You build this on Nasus you are going to deny yourself q stacks, while not having the tankyness or AP to compensate for the weaker q stacks. You do 3 things as Nasus: You destroy everything with q. You tank everything if you can't destroy everything with q. You build AP and get triple to quadra kills by playing like Xerath and landing as many E's as you can before you or your team fully commits.
If you do anything else as Nasus, you're not playing Nasus to his best ability and you are not utilizing Ravenous Hydra on the proper champ. You want Rav Hydra top great it works wonderfully on alot of champs. Not Nasus. You wanna go Nasus top, the best way to go about it is to build completely tanky and max CDR. Let your q be your only source of dmg. At around 350 stacks your dmg output will be vastly superior to any ad or ap build. Nasus is a scaling champ. You grow his strength.
Building Hydra is a set amount of flat offensive increase, and thus inhibits Nasus to operate to his own kit mechanics. It's like building AP Vayne. You need to stop.