Of all the dangerous devices to disassemble, they're one of the safest. A phone charger might still have 400V across one of the capacitors ten minutes after unplugging it (if you're in a 230V RMS country, so have more than 400V peak-to-peak), but a microwave's high-voltage section is only powered when it's plugged in, and microwaves are so long wavelength that even if you reassemble the waveguide or outer case badly and leave gaps, there probably won't be dangerous levels of microwaves escaping as gaps much smaller than the wavelength in question don't compromise the Faraday cage.