In another 5-8 years this will be perfected. Right now as it stands, there is about an 8 second lag time of no communications when it switches back and forth. It will solve A LOT of problems, just not perfect yet.
The current code base requires 30 seconds of -126dBm or worse RSSI and CC Scan for a LTE/WiFi BeON roam to occur at which case you have wait for the CCM to power on which can take another 15 seconds or if you have FW19+ you can set it to always power on (which wastes a lot of battery) and it still takes at least 5-10 seconds for VIDA registration and then Profile provision to occur. So in total a user is looking at 35-40+ seconds for a complete broadband cutover on a XL radio. And that is assuming that the radio does not re-acquire the CC and restart the 30 second timer.
Smartconnect has 100% the leg up on registration and failover over BeON, we shall see what they change when XL-Connect comes out as the next iteration of Harris broadband.
Back on the subject of Multi-Band Multi-Protocol Radios, the tait radios cannot do multi-mode without "switching" modes so as to say you cant scan DMR and P25 and use them both quickly. The VP8000 seems to be the king of the castle right now when it comes to working across bands and protocols as it stands right now.
As many know there is development in the works for possible multi-mode support on the Motorola P25 platform but when and if we see it is still a big question. Not to mention the cost associated with it if it does matriculate.