The systemd unit files should support the reload action so that the daemon can be HUP'd when config files are updated. The SysV init scripts supports reloading and the systemd files should too.
The architectural changes required for HUP to work correctly are massive. They're only likely to be able in v4.0.x even then you might be waiting until v4.2.x. Until it can work properly, in a way that everyone expects, FreeRADIUS should be restarted not reloaded.
I've been using reload with 3.0.x and it seems to work. At least I haven't noticed any issues with my usage. Looking forward to proper reload support.