On the plus side: its free and easy to do, and even if it does not come close to the direct listening experience (and cannot be used for strict A/B comparison), it can give a rough idea of the sonic signature of each system. On the minus side - all the things mentioned above.
Two dilemmas: 1) I can get better recordings using Rode mics into a Steinberg UR22. However, then I would not follow the mobile "standard", it would depend on the others using the same or a very similar recording setup.
2) Comparing system A and B by playing the B recording through A is obviously not ideal, since the B sound will be ’tainted’ by the A sound. In fact its amazing that the differences are notable at all. Yet I hear them. My friend with the B system has much more costly speaker drivers than I have, and I can hear it, especially in the treble. I think this is due to the fact that the sound changes in the mid region, which the mobile pics up, although it cannot fully capture low bass or high treble. In order to avoid the two-in-one problem I've used another system C to test A and B recordings. C is a small (OTL + single driver speakers) home office system. In practice, I hear much the same differences between A and B whether I play them on system C or A (except for bass, which is limited on C).
One more minus: on some phones, including my p30pro, the sound is better in video than in sound-only recordings. No idea why, but this phone is extremely focused on visual/camera performance, while the sound is almost like an afterthought (e g it only records in mono). I see that the video recordings can be 100mb or so for a couple of minutes, while the sound-only is 1 or 2mb. So, clearly, a much lower level sound format. Further, converting the large mp4 video file into a sound-only file (as lossless as possible) is a problem I haven’t solved yet.
