but have wondered if there is a real danger, or is it more just some "lawyerly" precautions?
There is no real "danger" to you, only to your gun (and your wallet, maybe).
Now, old model 29s are not glass eggs that break if you look at them wrong, but they are not the strongest or most durable DA .44 Mags out there TODAY. Once upon a time, they were.
Also, in that long ago time, there were no 300gr bullets ultra heavy loads for them. And it was expected that it would be rare for anyone to fire more than a few hundred magnum loads a year.
And, for that, the 29 will give years of good service before needed a tune up. When people began to use those uber heavy loads regularly, or large amounts of the "standard" MAGNUM 240gr loads, Model 29s began needing to go to the shop more often.
Everything wears, and eventualy needs a tune up "adjustment" (or sometimes more). Shooting loads heavier than the design is stressed for means more frequent repair. S&W addressed the issue with the 29-5 model, which incorporated what they call the "durability package".
Basically, if you run your car to the red line evey time you drive it, you won't get 200,000 miles from the engine (or maybe the tranny) without serious overhaul. But if you only put your foot in it once in a while, it will last a lot longer.