It's very telling that they even mention "minimal color blur" at all in the description of the 100-400. Usually they don't mention any kind of fringing other than to say it's been fully corrected; of course that rarely means wholly corrected and we always have to remember that what claims are made in any manufacturer's marketing text are always exaggerations. That they would acknowledge this lens has "minimal" fringing in their marketing speak means, by real world terms, fringing must be fairly strong. Which itself is not at all unexpected for a cheaper zoom lens which has prioritised size and price over optics, but it
is surprising to see them feel their marketing had to pay lip service to it.
"Capturing starscapes" sounds promising. I'll still wait for the reviews. With the RF mount, Canon tends to cut corners sometimes and rely on digital distortion correction.
Given how small it is, distortion is a given, but worse for astro is that it will be extremely, extremely shocking if the 16mm has anything
less than 3 stops of vignetting. Bear in mind much larger RF lenses have come out with corners and sides 4 stops darker than the center, and the nearest EF prime, the 20mm f/2.8, has over 4 stops of vignette even despite being a much larger lens and not going as wide. Even regardless of the potential distortion, the kind of strong vignette that Canon has always tolerated on its wide primes, and the
increased vignetting they're tolerating on RF lenses, means most of this lens is going to be, what, t/5-5.6 on average across the frame; probably t/8 to the sides, maybe even close to t/11 in the deepest corners?
All-in-all, I still expect these two lenses to be perfectly servicable hobbyist lenses for anyone who wants or needs to prioritise size and cost over quality, but with every release it becomes increasingly clear that the standard for RF is going to be a heavily reliance on software correction for any non-L lens. The end result can still be fine—the Fuji XF line has hinged on opcodes and software for the last decade, and that system is perfectly good for most tasks—just nobody should be expecting corners to
not be cut in the process.