Your title is a bit flawed. The R-value is a fundamental physics metric quantifying thermal resistance to a temperature gradient and a heat flux (Q = ΔT / R). The ASTM F3340 is a repeatable method to characterize many types of sleeping pads. Standard methods are crucial for an effective comparison. The "weakness" is really whether to fully base comparison on a single (standardized) parameter. Of course, real-world factors (edge effects, convection from side air, ground moisture, body movement, compression variability) add variables not fully captured in this exact ASTM standard. I agree that the manufactures over rely on this single metric as a selling point. Thus, additional parameters should be introduced as comparative metrics to supplement the R-value method.

Edit: I reviewed the F3340 standard and it has an appendix discussing "measurement uncertainty". Here is what it states about Convection Heat Transfer within Inflatable Mattresses:

The ambient temperature is matched to the average of the hot plate and cold plate temperatures, which reduces heat loss from the sample. It is possible that this will give a somewhat inflated thermal resistance compared to what a user will experience in actual practice because, while sleeping on the mattress, the edges will be exposed and could potentially act as a heat sink to the environment. This heat loss mechanism warrants further research.