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Can I ask you the direct question, based on your experience, are there factors specific to a FET that could potentially give rise to anomalous behaviour in its role as the oscillator in a GDO?
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I'm afraid I don't have any experience of tinkering inside FET or valve dip meters. However, I'd expect a valve to be more tolerant of the wide (amplitude?)range of waveforms that can be expected from a multi range oscillator like this. A JFET has the PN junction in the gate to consider. This may cause issues with getting a decent dip at low frequencies if this junction goes into conduction with (larger?) waveforms at low frequencies? A shunt diode can be used at the gate as a form of clamp but I don't know how effective that is in this case. But that is all just a guess and a lot also depends on what is used to meter the dip. A valve meter presumably looks for a dip in grid current. By contrast, I've seen solid state meters that just detect a dip in RF amplitude using a diode detector.
The only dip meter I have here is my old Maplin Precision Gold YN48C dip meter. I can't remember when I bought it but it was probably at their Southend branch in the 1980s. This is a fairly crude device that uses a BJT oscillator and a diode detector for the metering. It probably isn't the worst dip meter ever made but it is probably best described as 'adequate' on most of its ranges if operated with some patience. It isn't always easy to get a dip with it but perseverance and a few different approaches usually gets there in the end. Despite this, I have used it a lot over the years. In recent years I mainly use it as a quick and dirty signal injector although I have used it as a dip meter as well.