It's a good read, but I still maintain that the immovable zero page and stack make the '02 sub-optimal. The 816 lets you move both around, and the WDC C compiler at least does some nice things with this to allow a proper stack frame.
I suspect that an LLVM backend for the 816 would have to be something quite a bit different from the 02.
The downside of the 65x816 compared to the 65x02 is the address/data line multiplexing. In order to add 8 more address lines without going above 40 pins,[0] they multiplexed them onto the data lines. So to decode the address, you need some support circuitry for latching and gating. The 65x816 datasheet (from WDC) gives a schematic for doing so, but it’s not as simple/clean as a 65x02.
I personally would choose the 65x816 over the other for a new design, but I can understand why it’s not as popular.
[0]: 40 was the de facto maximum. Although, the M68k had 64. That thing was a monster in size.
I suspect that an LLVM backend for the 816 would have to be something quite a bit different from the 02.