This combined A7+M4 device is unusual, but is simply a cost saving over what would simply be an A7 device serially attached to an M4 (for example). There does not seem to be any new features here other than the cost saving from combining them.
This report from 2016 puts NXP at twice the marketshare over STmicro who are sitting steadily in the fifth place:
https://epsnews.com/2017/05/01/nxp-tops-microcontroller-supp...
Although at least part of the sales volume difference can be explained by NXP having had more expensive i.MX range available (and others), so in terms of chips sold they might be closer to each other.
> This combined A7+M4 device is unusual
I'm not sure if I'd say that, previously NXP did the exact same thing with i.MX7, and TI with their AM5x. And of course don't forget the classic AM3x which had PRUs filling part of the same role.
And before that, they had their Vybrid line that's a A5 paired with an M4.
To be fair that is largely because they aqquired Freescale, the year before they had almost the same market share.
Didn't mean they're an underdog as such, one can still be up the top and push hard. Many just don't.
> the things you mention are pretty much par for the course
As I said my background was Arduino, and the ARM offering from Atmel is a lot more expensive from what I can gather. I don't know too much about the others. Coming from the Arduino, the peripherals of most STM32s are something else.
edit: of course the price point is also a big factor in this, with the "Blue Pill" and similar dev boars in the <$2 delivered range.