Powder migration may also be related to paper hulls and paper wads. Powder migrating around a plastic wad that fits the inside of the plastic hull very tightly is something I simply can't imagine.
It does happen with modern data/plastic wads/plastic hulls. The tell-tale is a "bulge" in either the powder/wad area or a "bulge" in the wad/shot area. Reloading shot shells is not as controllable as reloading brass cartridges. Generally, it's hard to miss a "bad" brass case with some experience and personal quality control. There are a
LOT of visually identical Winchester AA hulls, for example. Some are very difficult or virtually impossible to reload (age, wear, external/internal soil, etc.). Some have different internal volumes because of different production lots, aging, and reloading (in this context, reloading and aging tends to increase volume, production lots can go either way). The powder migration I have seen is mostly caused by storing partial boxes of cast-off reloads in a garage, shed, etc.
Back in the '70s, my father used to alter his loads around the aging/use of his Federal 12 ga hulls. I've shot and reloaded a lot of Federal hulls, and I don't see a big difference in 12 ga, per se. I tend to use Winchester in 12, Federal in 16, and Remington in 20. To be fair, I think this is more of a case of what I have, rather than any sort of systematic testing.
EDIT: I realized I didn't make myself clear. I have seen powder migration caused by the occasional "bad" hull. Most powder migration I have seen seems to have been caused by bad storage, such as high heat/humidity. I don't believe that you can obtain a perfect components combination, because even with a perfect powder/wad/shot combination, you will get a "perfect" hull that decides to split, separate at the brass, etc. Like I have said with brass center-fire cartridges, you are probably best served by only dealing with your own once-fired cases/hulls.
