kexec is great, but has one significant shortcoming: some hardware and/or drivers do not handle kexec properly. In particular, various graphics drivers had this issue that after a kexec apparently some state was carried over and the display was garbled. Because of such shortcomings, kexec was never the default. But if you test it and it works properly on your machine, then great, by all means use it. It is also possible that the quality of hardware, firmware, and drivers has improved, and we could recommend it more widely, but we would need to back that up by widespread testing.
I think we could expose this as an option in /etc/systemd/sleep.conf. We configure various aspects of how suspend and hiberation works there, so also controlling details of reboot would be a natural fit.