My VIC-20 was the first retr0brite I tried. Turned out fine but we will see if it sticks. I used liquid just like @Arnuphis in a tub, left it in the sun for a few hours.
Now, my attempt at doing a retr0brite on the keys, THAT was a disaster. I used cream and the outcome was discoloration of the brown. So, lesson learned on that.
Currently working on a retr0brite of a calculator body for my brother. He asked me to give it a try, even after I explained the possible issues.
WRT brittleness. I, in my very limited experience, have not found the plastics to be brittle - at least for the age. My guess is that the plastics that are yellowed also suffered structurally from the same environmental stress, and the assumption is that the process made them brittle, when in reality they were already brittle.
Looks great (except for the above mentioned key botch) and runs great. I recapped it, but I'm toying with the idea of doing the S-Video mod. Not 100% sure I want to go that route yet. I did get the Penultimate+ cart, love that thing! Oh and the spring I lost on the keyboard; got a replacement in and I'm whole again!