They're not hobbyist friendly - 16x16 'tiny' BGA packages, so there's no real benefit over other ARM SBCs, they're massively slower than almost everything else atm. The advantage of STM32 was always that you could prototype on a SBC, but the chips were accessible enough to use in hobbyist projects.
Although since I recently discovered [1] that the R-Pi has an exposed memory interface bus (Secondary Memory Interface, SMI) on its GPIO, I’m far more inclined to use them instead. Getting low-latency/high bandwidth I/O into these “A” (rather than “M”) processors has always been the thing that made using an I.mxRT, STM MP1 or Zynq attractive to me. Turns out the lowly Pi has had that capability all along…
Coupling a Pi via SMI with a low-end (read: cheap) FPGA (Ice40K, or Efinix, hell go to town and get an Efinix T20 with 195 io for $8) makes a potent device, IMHO.
You can still get an rpi zero W for $15 [2], and even the quad-core zero 2W if you shop around is ~22€ [3], which blows the STM away in terms of CPU, video out, ease of development etc.
They’re not for hobbyist. Wrong shelf. They’re for real products and not SBCs. That’s why you also don’t have NXP parts in the hobby area. For low volume you buy SOMs and for big runs design complete printed circuit boards.
