No full range driver is really full range.
Some can do alright in the highs, like the Tangband 1772 and 1808.
Whizzer cones can be problematic due to breakups and a bit of discontinuity with the main cone. PHY gets around this by eliminating the whizzer cone on their 6" driver and having otherwise low mass. It has surprisingly smooth highs.
The big problem you run into is beaminess. The solution is either to crossover the driver into a tweeter, or add a rear firing tweeter (and not cross over the main driver) to correct the tonality and help with soundstage palpability.
If you have that sorted, then the other issue is bass. The trick here is to get the driver to go down to something well less than 80Hz. If you can do that, then a sub can be used that has no output (including distortion) above 80Hz. If this is done the sub will not attract attention to itself. I prefer the Swarm subwoofer system from Audiokinesis, as in addition to being a worthy sub (goes to 20Hz) but also solves standing wave issues in the room so bass is evenly distributed, especially at the listening chair.
The other limitation most 'full range' drivers have is power is they are also efficient (+95dB) - most don't handle a lot of power, the ZU being one of the exceptions. That little 6" PHY I mentioned would be toast by the time it sees 20watts, and its rated at 96dB. In most rooms 10 watts works with it, as long as a sub is also involved. If you plan to use an amp that has more power than the driver has, its a really good idea to place a fuse in series with the driver! Some of these drivers are quite pricey owing to the level of precision needed to make them work. Frying a voice coil can be really disappointing.
If you don't cross the bass out of the driver, you can expect Doppler Effect distortion and unnecessary heating of the voice coil.
Getting a full range driver to go to about 50-60 Hz is doable (more like 65Hz if you're thinking about that 6" PHY...). Adding rear firing tweeters while otherwise not crossing over the main driver is also doable. If you sort this out properly, you can get excellent results.
