There is a current thread here some where about huge condensation inside a Spindrift sock, which is breathable! IOW, getting to the physics - vapor condenses into liquid when it reaches the dew point. The dew point can be a cold pad or a piece of breathable nylon or a car windshield or a fleece Shug style bib hanging in front of your face, or just your breath in plain air.
And then there is sweat, a completely different deal. Overheating, maybe even in just one area of the body, = sweat.
As to your 1st line you say:
I've read that a waterproof undercover (Silnylon) can cause condensation to form on/in the under quilt.
If the Sil-nylon is cold, as it will probably be if it is on the outside of your insulation, and your body vapor manages to travel down ward and hits it, you will have abundant condensation if all the conditions are right. And sometimes they will be. I have done it myself and ended up with a soaked HH OCF pad and sleeping bag both on the footend . So a suggestion is to keep all VBs- or at least the 1st VB your body vapor encounters, on the warm side of all insulation. Keep them warm. And if you over heat with too much insulation and sweat, console yourself with the idea that at least all that sweat is next to your skin rather than soaked up into your down insulation.
What UC are you using? Are you referring to the HHSS sil-nylon UC?