I have tried room correction measurements on 3 different Android devices and beside the microphone differences, the internal electronics vary as well. Each sweep below was made using the internal mic and an external UMM-6 USB mic:
REVVL 5G Phone- Brick wall cutoff 8kHz and above with both the internal mic and the external UMM-6 probably due to A/D conversion limits.
Google Pixel 3XL phone- +10db peak above 10kHz with internal mic but more reasonable looking result with the UMM-6 external mic.
Samsung Galaxy Tab A tablet- Broad 15db peak centered around 7kHz with internal mic but results comparable to REW measurements with UMM-6 external mic.
The UMM-6 measurements were made "uncalibrated" since WIIM does not yet support mic cal files. However, the difference between calibrated and uncalibrated measurements taken with REW show only fractional db variations. When comparing the WIIM graph as measured using the Samsung tablet and UMM-6 mic against the REW graph, there was pretty good correlation between the two. Granted, the WIIM graph resolution required some interpolation on my part, but results were withing a couple of db WIIM vs REW.
The filter coefficients produced by the WIIM app and REW were quite different, but appeared to achieve similar target curves. I used "flat" as the target. When I have time, I'll enter the REW coefficients into the WIIM PEQ filter and compare the two results through subjective listening.
Conclusions- It seems that Android cellphones (mics/ electronics) are optimized for speech, whereas the Samsung Android tablet showed the most promising results with an external mic. I haven't tested yet if I can load a correction into the WIIM and then do an EQ sweep with those corrections in place. It seems like the WIIM app sets the filters to flat each time a room correction is initiated. I continue to be amazed at how new features continue to be made available for this fine family of products! My latest is the WIIM Ultra and I also have the Pro Plus and Mini.