I have had an iPhone since 2012 and an Apple laptop since they were called PowerBooks. I am often in road warrior mode. No, it is not seamless. I have to restart it quite often. It is nowhere near as convenient and always on like it is with my Apple Watch and iPhone and iPad. And it burns two batteries at once.
I am convinced that it is the Qualcomm royalty situation that keeps Apple from shipping a cellular modem like Dell does.