As I mainly reverse automative hardware, I care about only Tricore, C167, SuperH, and PowerPC, in that order - which means Binary Ninja is out for me for the time being!
Thanks for the post though, as I did look into adding a new Architecture and the setup for defining a new ISA is much simpler than it is in even Ghidra/SLEIGH, so kudos to that. Maybe if I find myself with a lot of free time I will try adding something.