Which is more accurate, mechanical water meter or electronic water meter (electromagnetic flowmeter or ultrasonic flowmeter)?

Generally speaking, compared with the electronic water meter, the accuracy of the electronic water meter is better than that of the mechanical water meter, but neither the electromagnetic flowmeter nor the ultrasonic flowmeter you mentioned are water meters. If it is a flowmeter, its lower limit is relatively poor. When the traffic is small, it will not be displayed at all because it is out of range. So it's hard to say. Flowmeters are more suitable for measuring media with stable flow.

If the mechanical water meter is compared with the electromagnetic water meter, the electromagnetic water meter is definitely more accurate.

If you are interested in flowmeter and water meter, you can learn about TSD electromagnetic flowmeter and TSDS electromagnetic water meter.