For me, having a dedicated trainer setup is a massive plus - whether that’s achieved from having a smart bike or a permanently set up smart trainer plus dedicated trainer bike (“trainer mule”) that always remains attached to it.
That being the case, a question then is whether the trainer setup needs to be shared between people or whether in reality it’s a solo rig that will only, or almost only, be used by one person. Clearly, a smart bike offers faster switching between positions than a trainer mule, so if multi-user that’s a massive attraction.
For me, multi-user was a possible nice-have, not a must-have.
IMO a smart trainer + trainer mule combo offers a more “robust” and “maintainable” setup. What I mean with those quoted terms is that system failures can usually be resolved more easily and more quickly with less recourse to the manufacturer, without a single point of failure taking the whole thing out. Obviously, issues with the trainer mule bike can be readily fixed by pretty much anyone, while catastrophic failures with the trainer can be resolved if necessary by replacement with another trainer of the same or different brand, offering various options to get your setup going again, quickly.
Conversely, that is the weakness of the smart bikes: a great, no-maintenance option when working as designed, but being a monolithic system borderline catastrophe when they fail and you face the logistical issues and potentially lengthy wait while the issues are resolved, if you are unlucky.
My heart said smart bike, but my (risk manager) head said don’t rely upon remaining lucky, so for me a trainer + dedicated mule won out. I built the dedicated mule from a v cheap frame, inexpensive bits and a 105 mechanical groupset. This has worked well, and I perform a clean/wax lube on the derivetrain about once per month to keep it running sweetly. This route seemed the best option for me at the time, and with hindsight that remains the case.
Others with different requirements - particularly the frequent multi-user question - will reach different conclusions - that’s what makes a market! Figure out what the most important issues are for you and the answer of which route to go down should become much clearer.