I have never heard a hybrid electrostat sound as "one piece". But the sound of other drivers doesn't seem as egregious in other types of designs, to my ears.
Which drivers are harder to integrate? Which Easier?
In my opinion, you want to match up these two things:
1) Radiation pattern characteristics, particularly in the crossover region. Narrow-pattern line source electrostatic panel + wide-pattern point source woofer = worst case scenario, tonal balance will change with listening angle and with listening distance.
2) Thermal compression characteristics. Matching up efficiencies is a good starting point. If one of the drivers suffers from thermal modulation or thermal compression sooner than the other(s), you could end up with a tonal balance that changes with level.
I would rather have two drivers that play nicely together than a plasma tweeter and a conventional woofer, which hasn't a hope in hades of subjectively "keeping up" with the plasma tweeter and so the system will never sound coherent.