Disclaimer: Let me start by saying that I'm certainly not a synth god. I love sound design as much as the next person but more often than not I buy preset packs from others because I don't have the time to sit around doing something that I know others could do better than I could anyway.
What it boils down to is how the program can modify the waves it's using, the effects it has built in, matrix controls, LFOs, modulation, etc. and how the program itself works with the data it is told. For instance, in audio you can have two compressors that have the exact same variables on what it will do to the sound it's given. You can set these exactly the same in each and end up with completely different sounding results because of the way it emulates that sound in each. The same can be said for synthesizers.
Rapture could be good but it needs updated. I don't think there's been any updates on it in like 6 years. Someone might correct me on that one but I believe it's been that long if not longer. It also needs a massive interface update and those are things I'm not sure Cake wants to spend the time on now that Zeta+ 2 exists which in itself isn't a bad synth at all, but I find myself using it less and less for things like Serum.