There's nothing mysterious about the Allwinner boot process at least. While there's an on-chip boot ROM that can't be modified, it just loads a chunk of code into SRAM from the boot device and jumps to it at the highest privilege level supported by the hardware. It's not uncommon for everything after the boot ROM to be 100% open source code. That simply isn't possible on the Pi, which is why Debian doesn't support it out the box.
Also, the lack of power management on the Pi is mostly a hardware limitation. The hardware simply doesn't support any kind of software-controlled shutdown or suspend.
How is a blob not mysterious. The fact that it doesn't use some exotic gpu-based bootstrap is irrelevant. A black box is a black box. Also I'd argue it is uncommon for everything the kernel uses to be open source, I don't know of one where all the drivers are open source (without losing functionality).
The Allwinner BROM is about 32KB, almost completely reverse-engineered at least for the earlier chips, and in any case the only thing it does during a normal boot is to load a chunk of code from a fixed location on the SD card to a fixed location in SRAM and immediately jump to it. It's nothing the Raspberry Pi boot process, where even the initial boot ROM has FAT filesystem support and it loads a proprietary blob which runs continually on the "GPU" co-processor with full access to RAM handling various important runtime functionality.
Also, the lack of power management on the Pi is mostly a hardware limitation. The hardware simply doesn't support any kind of software-controlled shutdown or suspend.