Well, it's not really impossible. Honest. You MUST control the lens, however. It doesn't erase bad camera work.
I can testify that it is possible to shoot the "Roma" sort of video handheld with the OLY 5-axis feature. The main reason the Rome video works: You'll notice that the editing never shows a shot come to rest after a floating move.
Those are the moments when the stabilizer will create the visual artifacts of it's use that you've asked about. If those instances were left in the edit, it wouldn't look as impressive.
For instance, if you handheld pan to the left and then stop on a subject, the stabilizer system doesn't resolve this motion in a natural way. It looks mechanical as the movement ends and the frame image comes to rest.
But, in the Rome video, the editor just cuts away during continuous moves, so no worries. If you plan to shoot and edit in a similar way, you can create such a video quite easily and completely hand held.
I mean, I've been doing cool mini slider shots myself by leaning around corners/foreground elements and shifting my body around. I swear camera operation with the OM-D is practically like doing Tai Chi while pointing a camera at something. That's really the best way to explain it and it does work.
Also note that in the Rome video the footage was conformed from 30fps to 24fps, creating a gentle slow-mo effect that also smooths things out a bit.
Anyway, I spent about 20 minutes on a small boat floating around Sunda Kelapa harbor yesterday with my OM-D and the stuff looks like it could have been shot on the world's longest dolly track. For the right kind of shots, it really is that good.
You have to be smooth though to begin with! Accomplished camera work is still needed. You can't just fling the lens around and expect good results. The Dixieland video above is not that impressive to me because of this undisciplined shooting. Smart considered control is a must.
Caveat: Pushing the focal length above 60-ish-mm (Full Frame Equivalent) and emulating dolly moves gets a lot tricker than stuff shot with shorter focal lengths.
I will say that I can see myself using this camera WITH a glide-cam type of rig to accomplish incredibly controlled, elegant, and longer moving shots. It would closely rival the best Steady-Cam shots from the most skilled practitioners. The combo would be very complimentary. While I can do glide-cam stuff decent enough, I'm not a pro at it. The 5-axis would hide those slight flaws I'd otherwise create.
Now, if OLY is able to up it's specs with firmware or a new model with new/better tech, then it's really going to tempt traditional users away from their usual brands, I think. Not sure how long they'll have the 5-axis advantage, and if other manufacturers can eventually match it, but for me right now, today, it's serving a very pragmatic purpose. It fits the way I need to shoot for my stuff.