There are quite a few differences between the Felix and the Dart.
When the Dart first came to the UK, it always had a fixed suction slider control - not variable suction control and no 3 point height adjustment on the handle that the Felix enjoys. In addition the ET-1 for the Dart had no brush roll on/off button but simply a light indicator to show wrong brush roll or brush obstruction.
As soon as the Kombi Deluxe floor head was designed, it was used on the Dart 4 which ironically featured variable suction control, but still no brush roll on/off button and no safety stop on the hose release -after all, the simpler things get, the better they are for a commercial stage of cleaning.
Im not sure where you have read reports that carpets are healthier than hard floors though.
I can't imagine that can be possible. Hard floors aren't porous enough to trap dust mites, though they may be susceptible to the bugs within pet hair and associated bacteria - but any hot mop going over a hard floor can sanitize it - even better with a dry steam mop - and continuous sweeping with a suction only floor head or parquet floor head.
Don't forget, SEBO also produce the excellent DISCO floor polisher for the Felix/Dart. It was developed to polish a hard floor without actually washing it. SEBO offer 4 different cleaning disks.
Extreme Makeover U.S often helps out families who have dust allergies. Carpets are not installed, but natural wood or tile is used.
Also an indoor air system that constantly purifies the air and keeps dust back is also installed. I welcome that in the UK home eventually, rather than rely on whatever comes next - antibacterial carpets? I shudder to think what extra maintenance would be required there.