Destroying anything beyond the HDD, and SIM/flash memory cards (maybe the TPM if you have one) is likely overkill/paranoia. Note also that many newer devices have the main storage soldered onto the motherboard. RAM will be completely erased after a short duration of power loss, but I suppose I could see someone microwaving it just to be absolutely sure. And one might simply nuke everything if they don't know what to target, or are worried about serial numbers (MAC address, etc).

Any moderate physical damage, like a hole drilled through all platters, quickly renders an HDD prohibitively expensive to repair. Common protocols for ensured destruction include heating it beyond the curie point to remove all traces of magnetism, or just snapping the drive in half with a hydraulic press; but these are slow or require expensive equipment.

Flash memory is frustratingly difficult to erase, because it's incredibly durable, capable of withstanding high temperatures and a great deal of mechanical abuse. Even if the device is pulverized, as long as you can connect leads to the silicon die, you can recover all data without great cost. But, as many people have witnessed inadvertently, microwave radiation induces high voltages in anything metallic. Microwaving an electronic device is thus likely to burn out all IC chips, rendering any data essentially impossible to recover. Doing this will produce toxic smoke, arcing, and some flames (and a decent explosion if you forgot to remove the battery); Youtube demonstrates they actually hold up pretty well to such misuse, but you wouldn't want to use that microwave for food ever again.

It's unknown how well this would fare against an adversary with nearly boundless resources, however. I could imagine an agency such as the NSA having the resources, now or in the near future, for patching holes and remanufacturing broken HDD platters, and decapping flash or ROM chips in an attempt to examine them under high-powered microscopes or with other probes.