Single driver speakers. Are they worth considering ?

This is another coaxial speaker; there is an add on tweeter ("super-tweeter" per Zu). The main driver functions to slightly above 10KHz.

No crossover (though filters). Still not a true full range single driver speaker.

Zu make full range single driver speakers.

With one driver we do not have any phase, timing and tonal discrepancy issues that plague multi-driver systems. There are no obscured bands in the midrange, hence there is a possibility to integrate the most sensitive midrange better to the room acoustics. When the integration to room acoustics is perfect, the jump is significant. With a well integrated multi-driver system the improvement is not as drastic, as the midrange is still plagued by issues from the multi-driver solution. The question is: do we want pure midrange (single driver) or extended extremes (multi driver)? Given that over 90% of our brain's sound processing power is in the midrange, we benefit most from improved midrange. We have a culture that obsesses with extremes, so most people go for that automatically. It's ultimately a personal choice. Just an observation: my friends who settled on a single driver solution have stuck to it, and rave about the music they are playing, and have no thought about speaker upgrades.... single driver sound has a rightness to it that multi driver cannot match. If you look for "submarine crashing to the iceberg" then they are not the right choice.

The OP asked a question, without specifics as to what is meant by "single driver" and "fullrange speaker". Seems like most responses discuss a dynamic speaker as a "single driver", and full-range speaker as s comparative well-known or accepted model that reproduces a wide enough audio frequency range to be called "fullrange".

My take on the question is a single radiating surface unit (speaker) that can come closest to reproducing the full range of audio frequencies (20 Hz to 20 kHz). Nowhere in the question is the required levels of distortion, power handling, efficiency, waveform accuracy, dynamic compression, linearity, sound coverage (dispersion), etc. 

Musical Affairs Grand Crescendo with 10 watts of SET is very nice.  Of course its not going to sound anything like Magico M with Soulution monoblocks.  Both are perfectly valid for enjoying music.