Regarding scope of embedded systems/firmware
When we released Software Development, I made a push to make all forms of embedded systems programming off-topic and re-direct such to this site.
The reason for this is is my experience from the "embedded" tag over at SO, where I'm one of the top users and pretty much the only one that has been consistently moderating that tag over the years. That tag attracts a whole lot of bad or off-topic questions.
Embedded systems trouble-shooting or review questions asked at a pure software site almost always end up in the borderland between software and hardware. It is very common that you don't know if the reason for the problem is located in software or hardware. And then it is not sufficient just to view the source code to find the problem, you'll need datasheets and schematics too. Quite often the problem is related to how a certain data bus is used.
Also, if the problem happens to be the software, the question is on-topic for the programming site. But if the problem was hardware, then suddenly it is off-topic.
So these kind of questions were always best to ask at the Electrical Engineering sites, where firmware development (microcontrollers, HDL, PLC etc) is on-topic.
This means that code review questions of firmware are also best asked at the Electrical Engineering site.
I've been participating in SE's Code Review site too, and whenever embedded systems questions pop up there, you tend to get confused PC programmers chiming in, coming up with advise that ranges from mildly useful to horribly wrong.
Generally, I think all forms of design review of electronics should be on-topic here. Schematics, component choice, PCB layout, real-time requirements, source code.
(Take for example someone asking for a review of their switch debouncing. They are using RC low pass filters as well as a software filter. How do you review that overall design without viewing schematics + code both?)
So if we are to make a new category, it should perhaps be named Design reviews and not just restricted to source code, but to any form of design reviews. I'd love to see more design reviews of schematics and layouts on this site too.