Why would it not surpass a conventional 3 way?
Seems to me it has advantages over such- no cabinet, no crossover ( other than the cap on the tweeter), and the vast majority of the frequency range is covered by a single driver.
A conventional design uses drivers optimised to cover the low, mid and high frequencies. They tend to achieve the best that can be done in terms of (technical!) sound quality within budgetary and similar constraints.
Stretching a midrange driver to cover the low and high frequencies compromises the design significantly not only for the high and low frequencies but also for the midrange. To cover the low frequencies the cone needs to be large and needs to move a significant amount in order to displace enough air to be loud enough. To cover the high frequencies the cone needs to be small and light. Some serious compromises have to made if the cone of a wideband driver is going to work to some extent and we can certainly forget any question of working well.
What tends to be done is to make the driver a bit too small for a woofer, with too small a displacement for a woofer, with too light a coil for a woofer and with too light a cone for a woofer. The low frequency response will be too quiet and not deep enough but apart from these significant failings OK. The light cone will resonate over most of the passband which is not a good idea in general but is necessary in order to have reasonable output through the midrange and into the high frequency range. As the frequency is raised the inner part of the cone moves while the outer part progressively remains largely stationary apart from some resonant motions. The resulting resonant ragged response is part of the characteristic sound of wideband drivers that is not present in anything like the same degree in competent multi-driver speakers. Some PA speakers have a similar characteristic sound due to their use of light cones for reasons of high efficiency.
As the frequency is raised and increasingly only the inner part of the cone moves the directivity narrows. This is another characteristic sound that differs from that of most multi-driver speakers. However, in this case if not too pronounced it may well be more of a positive than a negative when listening to music in typical rooms. Panel speakers like Quad electrostatics often have a similar directivity characteristic.
Having a single cone for low and high low frequencies will create relatively high levels of distortion as the low frequency motion modulates the high frequency motion. This will be more pronounced at higher SPLs where wideband drivers are usually limited and for lower frequency extensions where again wideband drivers are usually limited. Adding a sub and removing the low frequency motion from the main speakers cleans up the midrange and is one of the significant benefits of adding a subwoofer. A single cone for the full frequency range is not a good idea when it comes to distortion.
A single sound source may have some desirable characteristics compared to non-coaxial multi-driver speakers. Most multi-driver speakers have their drivers aligned in the vertical plane leading to there not being any significant differences in the usually more important horizontal plane compared to a wideband driver. In the vertical plane however the multi-driver speaker will generally have constructive and destructive interference in the crossover regions for the off-axis radiation leading to a degree of comb-filtering being present in the reflections off the ceiling and floor. It is an advantage but a modest one which is also shared by coaxial multi-drivers which are likely to technically outperform a wideband driver in other respects.
If the speaker with a wideband driver is modestly priced, you're not bothered by a lack of low frequencies, listen at quiet rather than standard levels, have a preference for the narrowing directivity, perceive the ragged resonant response as exciting, etc... then they could be worth a listen.
