My personal experience is two part. PVC does get brittle across time whether or not it is exposed to light. Sprinkler systems that my dad installed when I was a kid are now surprisingly easily broken when encountered with a shovel. As a result, I never use 1/2" pipe, and the couple times I have installed new residential water mains, I have gone 1.5" just so there is a bit more plastic there.
The second bit of wisdom, not all PVC is created equal. I have seen fittings stored in a drawer in the garage start to yellow over time. I have seen fittings in use but exposed to sun, in a period of just a couple years, become so brittle they shatter if you hit them with a hammer, leaving the underlying pipe undamaged. That shouldn't be possible. As a result I only buy fittings from recognized brands such as NIBCO, DURA and LASCO. LASCO is my long time favorite. I take the same approach to the pipe itself. Touch enough of it and you can tell, some just doesn't feel right. Fortunately, I really haven't seen the bad stuff in a while. Maybe we got smart and stopped importing it.
In your specific situation, if it was outdoors, I'd relegate it to use in sprinkler systems (which is what I do with my older parts) and perhaps drains, as you suggested. We don't use PVC for drains here, so I don't come across larger fittings very often.
Basically what it comes down to for me, if the quality of the work matters, why would you want to risk trouble with an unknown fitting? Fittings are cheap compared with the potential headache of a failure.
I should add, there are three types of pipe that I deal with here. The traditional white PVC without UV inhibitors and the grey (typically for electrical work) and 'pinky beige' varieties with UV inhibitors. The 'pinky beige' is much less common but used for above ground, generally temporary, sprinkler systems and water supply lines. I only mention it because it could easily be mistaken for 'sunburned' white. There is also purple for recycled water, but I doubt you'll be coming across much of that.
