These cheap high resolution panel meters usually use an MCP3421 ADC and µC. So there are not that many parts to drift: mainly the ADC internal reference and the shunt. Both would be relatively small changes in the scale factor, but not offsets. So not a big deal for moderate accuracy over a large dynamic range. There should be low offset drift, which would be the critical factor.
However the ready made panel meters may need an isolated independent supply.