That isn't to say there weren't expressive analog polys. I'd point to the CS-80 as an expressive instrument, with polyphonic aftertouch, ribbon controller and lovely ring modulator. It is just that subtractive offers a fairly limited set of parameters suitable for expression. Once you've routed aftertouch or velocity to open up the filter or amplifier a bit to make things louder and/or brighter, you've pretty much exhausted much of what makes sense musically.
While it is true the DX7 justifiably earned its reputation as difficult to program, the one thing people seem to neglect to mention is Yamaha's implementation of FM synthesis was extremely musically expressive to play. The patches came alive with velocity, not simply brighter or louder with velocity. The timbre could change is ways that were musically sensible, but responded with shifts of timbre way beyond what was possible with subtractive. Aftertouch was standard. The modulation wheel could do many more interesting things beyond introducing vibrato and if that wasn't enough, there were foot and breath controller inputs. All of these could produce timberal shifts that were previously unheard of.
Every major player needed their own take on a unique digital polyphonic synthesizer and the K5 was Kawai's first real additive synth. The styling of the K5 is very late-80s Roland, bearing striking similarities to the S50 sampler.
This was the grandpa of the K5000 series which came much later. The K5 retained all the qualities that made the DX7 difficult to program, but the real time expression didn't extend beyond what was typically available on a subtractive poly.The result was a very thin, flat, glassy synth. Worse, it was fairly noisy.
I picked up a K5m in the early 90s for $150, and, despite the limitations mentioned above, I used it a ton. I had a small program that would analyze Mirage samples and send the resyntheized data to the K5 for an extremely poor man's Axcel. It had a very capable multi mode. While programming individual harmonics over time (well, there were only four envelopes available to control the harmonics) from the front panel isn't very much fun, or musically intuitive, navigating and altering the multimode was straightforward and easy to use. So, I used it in layers a lot, adding a digital sheen to other, fatter synths, like the K3m.
The K3 predated the K5 by a very short amount, and it was pretty much a standard subtractive synth. The K5 did have a few unique features for the time, including a limited waveform creation mode where you could build up a waveform, harmonic by harmonic that pointed gently at where the K5 was ultimately going. There were a total of 32 different available waveforms available and these were fed into a lovely analog SSM filter. The envelopes were also analog. In mono mode, the K3 was completely capable of gobbling your entire dynamic range. The K3m was another module that I picked up used for $150 and used for a number of years.
Both the K5m and K3m ultimately gave way to a pair of Kurzweil K2500s and Roland JV1080 that I used from the mid 90s on though a transition to software instruments and a shift to the modular as the dominant hardware synth.
Update. I managed to dig up a very early specimin that is exclusively K5 and K3. The K3m is providing the bass, everything else is K5m. ORIGINALLY RECORDED TWENTY YEARS AGO.
K3m K5m example by stretta samples
