Update 2 I did a quick test with a SanDisk sdcard that my phone has tested to be writeable at 30 MB/s. It is more expensive than the others and should do better.
At 2 pic/s - an image everyh 0.5s - the Xiaomi YI skipped an image about every 30 seconds. I tested for 1½ minutes and I found 3 missed images. Each file was less than 3 MB in size (12 mega pixels). I have not tried with lower resolutions - it may just make the difference.
I think this is good enough for Mapillary, especially when shooting from a car or motor bike. Mapillary can handle the skipped images file. It would not be good if you want to do a time lapse, but they are usually done at at slower framerate.
Update 3 I have written a post about my experience and research on microSD cards - don't buy to cheap .
Update 4 I tried with a new Samsung card, that advertise write speeds up to 90 MB/s. The 64 GB were filled in about 4½ hours with 30.000 images at 8 mega pixels. At 0.5s there still are a few holes, where the camera has skipped an image. Perfectly okay for Mapillary - not if you are making a timelapse movie.
The images are around 2 MB in size, so I will conclude with certainty that it is not the write speed of the cards that causes the gaps: It is the camera itself. But for Mapillary gaps are OK and when going by car 0.5s is great! I am just happy that Xiaomi gave the option!!
Oddly enough, the camera stopped when there were 435 MB left on the card.