Intensity Z-projections are very often the go-to method for many scientists with 3D images, since the additional dimension makes the analysis more complicated and many struggle with an analysis in 3 dimensions. However, Z-projections are mostly not a good idea, because they neglect the third dimension and therefore often lead to inaccurate of wrong measurements. This is especially true for pixel intensity measurements.
Here, you see an explanation, why this is the case!