With respect to "ease of drive", I looked at the CS1.5 which I believe do not use "Thiel designed" drivers but more or less "off the shelf". Based on Stereophile measurements, the impedance minimum is about ~3.7 Ohm at around 200Hz. Based on that, I assume the drivers used in the CS1.5 are 4 Ohm design. The overall impedance of the CS1.5 is not that different from most 4Ohm speakers using conventional design approach.
For comparison, I ran a quick simulation on my time-phase coherent design, all using 4Ohm drivers:
1. For a 2-way design, minimum impedance is 3.6 Ohm at also 200Hz.
2. For a 3-way design, minimum impedance is 3.3 Ohm at around 160Hz.
But using a more conventional NON time phase coherent methods, the impedance is about the same, therefore my guess is time-phase coherent is not the real reason for being "difficult to drive".