Yeah, that and/or a mkII keyboard version with more dedicated multiengine controls and the simple sequencer from the XD (and both desktop and keyboard versions).
They seem to have all the parts figured out but just don’t see that as a successful instrument - I don’t get it.
I would not be upset about such a MKii if it released at a higher price point, but maybe those skilled hands are all working on the PS-3300 (which is probably a bit rich for my blood, and too big).
I’m so happy to have the prologue 8.
And confused Korg has not figured a way to make a new baby like this one with 6 or 8 voice.
The monologue xd with it’s 4 voice, it’s a bit too low in voice count, so hope my prologue won’t break 
And if it break would replace it by a XD or a used prologue.
And if a mk2 version came one day, hope there will be a more contrôlable HPF. And more slot for the user oscillators, mod and reverb/delay slot.
I think it’s quite reliable, but I had few issue with some korg products between 2000-2020 related to ms2000, zero8 mixer.
For programming custom user oscillators, the best would be to look at the GitHub there is some example code.
It’s basically really a barebone « c » code using float. So if you know how to write an oscillator in « c » you should find your path easily.
I’ve just learned that there is a SDK development board for the Prologue multi engine that they sold at superbooth 2018 in very low quantity. Quite interesting.
(This is not my offer)
I’ve also seen that, what would you do with this? Didn’t people design custom oscillators and effect without this? Is the only advantage that you don’t need to transfer stuff to an actual Prologue to test things?
There is different target board for the Korg SDK. No idea what would be the use of a standalone board specific to the prologue.
The only use I would see is : I guess you might be able to easily software crash or probably corrupt some data on the prologue. And a reboot should solve the issue.
So maybe this board allow you to test your code in a more safer environment. And when you feel ready you can upload to the prologue.
On my side, if I jump on coding on that I will directly upload to the prologue, no safety net 
Probably at the time not many Prologues were available and they wanted to give developers the opportunity to start with this.
Yeah the modfx slot can be switched with user filters. Have a few but not a comb one. Some cool ones with lfos on the filter to allow you to use the single lfo for other duties.
Fresh release candidate of Plaits user osc with front panel controls by John Walton
Sound demos: