Hi 
I want to add something from my experience to this aspect:
After searching on beatport and tunebat, I found that both keys determined by Engine DJ were incorrect.
I actually determine the key of the tracks manually, I listen and I play along on a keyboard synth or my piano. While I frequently disagree with the results of Engine DJ, I can now say that both Beatport and Tunebat are not reliable sources for correct keys. I do not know how they arrive at their results, but so far they have been wrong pretty much every time when I looked up their results. Listen for example to this track here:
Beatport says that this is in F major or 7B in Camelot notation, Tunebat says it is 2A or E♭ minor … but playing either of these scales over the track sounds quite a bit off. First of all, is this really major? Does this sound happy in any way? No, I would not say that. F sounds like the harmonic center note, I would say it is 4A or F natural minor … The notes A, D and E are in the F major scale, playing them along the track sounds not right. Now play a B, which is in the E flat minor scale, it sounds out of place as well.
There is some subjectivity involved here, the subjective nature of key identification, especially in electronic music where tonalities can be complex and unconventional. Though after analysing several hundred tracks from the Techno and Trance genres, I begin to see the ways in which Engine DJ deviates (which may of course be biased by the selection of my music collection and the psychological phenomenon known as confirmation bias):
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There seems to be a bias to assign a major Camelot key to minor tracks, if they are uptempo or energetic. For example we could get 9B or G major as a result, when in fact it is G natural minor or 6A. The harmonic center is identified correctly.
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Sometimes Engine DJ picks a neighboring key, for example, 4A instead of 5A. As these could mix well according to the Camelot wheel, we can expect these keys to have something that makes them compatible and similar. The mode is identified correctly.
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And sometimes Engine DJ picks a major scale from the outer Camelot ring, for example 10B or D major, when the tracks sounds much more like 10A or B minor.
… if it is madness, there is some method to it 
…