Although in general the harder the cone material, the more we have to do deal with frequency break up or frequency rise in speaker design, today, you can no longer make the broad statement about any specific tweeter design based on material used. Todays speakers have been designed with a multitude of damping materials and most of the phenomenon thought of in specific cone materials don't apply if a tweeter is well thought out in the design phase. Next, the minor problems that are not corrected in the tweeter design, can certainly be handled in the crossover. Any well thought out speaker today, should be fairly accurate with a minimum swing in frequency across its bandwidth.