All the talk in favor of glass carboys based upon "being able to see fermentation" doesn't make sense. Is seeing your fermentation a cool thing? Perhaps to some. But is it an advantage over not being able to see it through opaque plastic? No. For one thing, your beer shouldn't be exposed to light; it should be completely covered or placed in a dark place during ferm. So, there's no advantage to a glass carboy in that sense. Even if it is covered or in the dark, you shouldn't keep taking a peek at it in order to assess its progress --- that's done with a hydrometer regardless of the type of ferm vessel. Bubbling, not bubbling doesn't tell you a thing about the condition of your beer.
I love to watch my airlock bubble, but it's pure entertainment, not brewing science; seeing it tells me nothing.
I disagree, it's useful and not "pure entertainment"
"seeing" tells you a great deal - watching krausen rise and fall, and the yeast flocculate are all important pointers to me of different fermentation stages. You don't need to expose your entire fermenter to direct sunlight for entire fermentation duration to see this - just sneak a peak once in a while, that's super easy. I have fermented in the kegs but also in buckets and transparent fermenters (PET and glass) and there is no substitute for seeing the fermentation process.
I even have a brewometer so I can see specific gravity vs. time and I still get a lot of information from watching the fermenter.
Do you draw gravity samples daily? every other day?