They’re both different types of devices designed for rather different tasks. It’s true that they both deal with “parameters” at a basic level, but the Hydra really has nothing to do with VSTi plugins beyond that fact.
The Hydra Device is designed to link one input parameter to multiple output parameters and control them all simultaneously. It is not designed to let you control each output parameter individually, because what would be the point to that, when you can just control the actual DSP device parameter itself directly in the normal way?
The Automation Device is designed to take the internal parameters of a VSTi instrument, which are normally not accessible, and then make them available to you in the form of standard Renoise parameters which can be linked to all of the other meta devices.
If we take the Hydra Device and add the ability to bind it to a VSTi, it is still not the same as the Automation Device, because the Hydra does not allow individual control over the output parameters. If I’m controlling a VSTi, then I might want to automate 8 different parameters all at different times. How am I supposed to do that with the Hydra, when it only offers 1 input parameter that I can automate? The Automation Device, on the other hand, would allow access to all 8 parameters I’m interested in.
If I were in a situation where I wanted to automate 2 or 3 VSTi parameters all at once from a single control - such as filter cutoff and resonance, for example - then I would use the Hydra Device (connected to an Automation Device, of course) for this, because that’s exactly its purpose - the “one to many” concept. But again, this has nothing to do with VSTi, and those 2 or 3 parameters could effectively be anything I want.
PS… re: grouping multiple devices into one combined device that is easier to manage, I am 100% behind this idea 
See some other threads, such as this one
.