Honestly, it's a myth that mechanical are somehow inherently more accurate than electrical. In both cases you are taking a physical property like pressure of temperature and converting it to a gauge needle position. The accuracy is only as good as the measurement and the conversion. Every modern car uses electrical/electronic gauges for everything, including the speedo which has legal requirements for accuracy. So the accuracy of your gauge is down to the quality of the design & construction more so than the technology. Also, if we take a step backwards, how accurate do we need most gauges to be anyway? Realistically if they are within 5% of reality that's enough in most applications, and from a design/manufacturing perspective that is not really challenging.
In this day and age I wouldn't even consider a mechanical gauge unless there was some nostalgic appeal or you already had them for free.