This is actually bad news because it means SailfishOS programmers had to implement something different than the standard 20/23Mp modes.

A bit of math:
- Xperia X with Android sports "20Mp" in 16:9 at 5984x3366 resolution (20.14 millions pixels)
- Xperia X with Sailfish sports "16Mp", that is:
16 million pixels area in a 16:9 grid = 16Mp / (16*9) = 333² = about 5328x3000 resolution.

That is, about 12% less pixels on the X axis (~650 pixels less than in the 20Mp mode) and on the Y axis (~360 pixels less). I hope this difference is intentional and actually required to do the serious stuff before saving the jpeg file (only Cbeta guys can confirm the resulting quality), as ignoring outmost stripes from the captured image may help improve white balance. But clearly it means that Sailfish does "less than" the Sony-supplied camera software.

Having "many megapixels" is useless if the camera software saves the image without doing proper noise reduction, contrast enhancement, ISO sensitivity boosting and so on - things that only the sensor manufacturer knows how to calibrate.

Note: the only reason to have "many megapixels" was reducing the picture size - say from 20 to 2 megapixels - to wipe out artifacts and noise, but it won't give you a "night mode" shot better than the original OEM-developed driver of the sensor. Shooting in full sun will always give good results - the real test field for a camera is in low light environments.