What's with the continued fears here? To me, this general attitude is not unlike the fellow in this thread who somehow thinks barely touching leaded solder will ruin his life. I am as protective of my old Macs as you could possibly imagine, and I don't want to die sooner than need be, but I don't think "living" is about staying inside the house with the doors and windows perpetually locked either. There are risks to everything in life, but soldering a few things here and there or RetroBrighting your vintage plastics is not the same as skydiving, folks.
So if we wish to talk about legitimate deyellowing fears, then "blooming" would be the primary one. But again, my method uses off-the-shelf 3% H202 which won't cause blooming even if you soak plastics in the liquid form for more than 24 hours. Blooming is caused by people who have zero patience. They want to see results in 1 hour, so they blast the plastics with bomb-making-grade 20-30% strength H202. That's just insane. Use the off-the-shelf H202 like I do and you will be fine. If you don't believe me, ask our very own LumpyDog, who is the man I credit for having gotten myself started in deyellowing with ultra-low-strength H202.
Deyellowing your vintage Mac's plastic will no more make it brittle or crack it than spraying it down with 409 (which I have been doing on Macs continually since 1984). It's not going to happen on plastics as thick as our Macs use. If you wish to prevent brittleness and cracking of polymers then stop time. Continual exposure to UV light and "the general aging process" are the real enemies here. But thankfully RetroBrighting lets you turn back the clock in terms restoring the surface color of plastics to its original state. Just follow my or Lumpy's recipe and give it a go. It really is worth it.