I don’t know serum, but I definitely agree that it’s more a set of building blocks than a “sounds” with that said, I found it much more usable to make string, and Wavestation type evolving sounds rather than Organ sounds, for example. For those sorts of sounds, something like the prophet 12 is a bit better.
The thing about the hydrsynth is that you can run audio rate signals into the CV inputs, so there’s a lot more flexibility than might initially appear.
Also, I found the CC implementation to be quite good, unlike the digitone which will choke if you start modulating a bunch of parameters in real time.