My take is that the simpler the design the better. The intrinsic problems of single driver systems are pretty obvious. Asking a single driver to reproduce the entire spectrum is simply not possible. Having said this the midrange of the some of the best single driver speakers I have heard were spectacular.
I am not an expert, but an extremely complex high order crossover with a great many components is precisely the direction I dont think one should go. A patently bad idea. The designers choice of drivers may necessitate such a design.
Crossovers are inherently subtractive.