Opticallimits tested he Sony FE 28mm f/2 and concluded:

The Sony FE 28mm f/2 may be a prime lens but it’s quite obvious that Sony had (or wanted) to cut a couple of corners in order to save costs. The resolution characteristic isn’t overly harmonious to say the least. The lens may be sharp at medium aperture settings but has a substantial border/corner weakness at f/2 and f/2.8. Of course, you may argue that this isn’t really overly important for shallow depth-of-field photography but it’s hardly impressive for a prime lens in any case. If you operate the camera with activated image auto-correction, you won’t notice drastic weaknesses beyond this. However, both the original image distortions as well as vignetting are on the high side. Lateral CAs aren’t all that hot either for a contemporary lens. The quality of the bokeh is one of the better aspects. The rendition of out-of-focus highlights is a little rough but the general blur is very smooth.

The lens may not feel like much due to its low weight but objectively there isn’t anything to complain about the build quality. An all-metal body and a smooth focusing ring are good base ingredients and adding a constant length as well as the dust- and moisture-sealing makes it a very decent package. The AF is also reasonably fast and, typical for most Sony lenses, noiseless. Sony skipped the implementation of an optical image stabilizer but this would have been unusual in this lens class anyway.

The reason for the existence of a prime lens should be that it offers something “more” over a zoom lens its focal length class. In the case of the Sony FE 28mm f/2, it’s about an extra stop or two of speed. The question is whether this alone can be a reason for buying the lens …