One of the very, very important factors to keep in mind is that the electronic units tend to drift, and need calibration to be reliable.

This is critical in the case of 'wet' sensors, like pH or redox (ORP) electrodes. It's important even in cases of 'color' readers. By contrast, $10 OTO / phenol red kit do NOT have to be calibrated regularly. (However, the color standards in some of the non-brand name units tend to be pretty variable.)

My experience with electronic testers, of many types, has been very consistent when these units are used outside the lab: they don't work well! That's not to say someone could not make a unit that did perform well, over time, and without frequent calibration. But I haven't heard of any such product yet.

Ben