I started building some of these (and others–there are circuits available on the web for nearly every synth function-- and many you didn’t know existed) modules about a year ago. I can’t believe I waited so long to try building these things myself- I had been playing analog synths for over 10 years.
It’s a difficult question to answer, that is, why some circuits are difficult–or more difficult than others. But mostly, I’d say it coimes down to complexity. The MFOS sequencer is probably the most complex of any od the PCBs they offer. So, it’s probably better to start practicing on something a little simpler. It’s easy to think you have everything soldered in correctly–or have the right components in the first place, and then later find that your circuit isn’t working. At that point, it can be extremely difficult (without electronics experience) to track down what you’ve done wrong. Especially so on a very complex board like the sequencer.
But, if you’re extremely careful, and don’t rush things, there’s no reason for anyone to think they can’t build these modules. At that point, you don’t have to wait for Moog to issue an ADSR or VCA or Pitch to Voltage Mooger. Just build it yourself. You can even design it to look as similar to your Moogers as you want ( mostly depending on how much money you want to spend).