Models do not account for all physical behavior, unless they have a lot of money behind some models.
What happens if the amplifier oscillates at 40 kHz for a half-second?
What happens at the resonant frequency of the driven element(s)
Even though it's low-Q, there's impedance peaks and valleys in speaker systems. What happens to the amp when the impedance of the speaker system at 10 Hz is closer to two ohms than to four?
OK,I'll play. In the interest of full disclosure I'm an electrical engineer and I've designed loudspeaker passive crossovers (professionally in a former life) for more than 40 years:
1) Even simple loudspeaker models account for all physical behavior that matters. It's electricity and acoustics and physics, it's not magic.
2) No audio amplifier should EVER go into oscillation, unless it is defective or there is an error in the design. This has absolutely noting to do with the loudspeaker load. Unless something has broken or you have designed an oscillator, an audio amp will never oscillate. Now if you are talking about parasitic oscillation because of a high capacitance output in an amplifier, then what will happen is distortion and excess heat. But again, a properly designed amplifier should never do this. And at 40KHz you will never hear it.
3) At Fs (resonant frequency) the driver can move most freely, meaning it is easy to push to Xmax. Electrically the impedance of the driver spikes and efficiency drops very low, so the signal strength at fs is low. This helps to protect the driver from over excursion. A properly designed Xover will prevent drivers from operating at or near Fs; except for the woofer, which can experience fs although the impact is generally limited by a combination of system design (Fc > Fs) and designed high-excursion capabilities. Some drivers (specifically ELF subwoofer systems like those from Bag End or Sun fire) operate exclusively below Fs, which means they must have high excursion capabilities and they consume large amounts of power to achieve usable output levels.
4) Again, a properly designed Xover will minimize the effects of the natural impedance swings across frequency of the overall system, no matter what might be happening in any specific driver. Unless you are listening to large pipe organ music or some EDM, you will never experience a signal at 10Hz, but if you do and if a loudspeaker system hit 2-ohms at 10Hz (unlikely) than the only thing that will happen is this: if the amplifier can handle the load, the power output will increase over what it was putting out at higher impedances. If the amplifier cannot handle the load, it will go into protection.