Maybe 5G itself is not really ready, but seems like Apple had deals with the carriers, thus the push for 5G now.
If it's not a hardware limitation, then maybe it's an artificial limitation put by carriers like Verizon so customers wanting full 5G performance (and why not? They already paid for it) must use the phone exclusively with that specific carrier. It could be something similar to some AT&T plans not allowing hotspot.
So could be real technical limitation, or Apple bending over for the carriers, or both. Thus the risk of being on the bleeding edge.
I remember the old days where dual SIM Android phones only support 3G/LTE on SIM1, while SIM2 is stuck on 2G, for so long. There's always an excuse like how dual 3G/4G stand-by modems will use too much power. It's not until countries start to shut down their 2G networks that magically the technology caught up. Now we have cheap Android phones capable dual VoLTE on both SIM 1 and 2. Magic.