As ever it depends which style we're talking, but regardless of style there will always be the presence of musical phrases in the music with various cycle lengths and with varying degrees of complexity. People coming from a musical or music production background would probably be more readily able to identify these for even greater DJ mixing ability. Then of course you also have those coming from a purely DJ background (of a progressive/blended mix style) who might be a little more observant who will also suss all the a fore mentioned out for themselves too to then be able to use it to their advantage despite maybe not having the means to describe or explain what they do to just do it when doing it. You'll also find a lot of these DJ's eventually moving into production themselves. 

The last time I used the SYNC button on Traktor and the Pacemaker it was often the case that it just wouldn't actually cue up at the exact place (or particular point of a musical phrase cycle) that I actually wanted to cue up at despite possibly initially being perfectly matched in tempo and perfectly on beat to still sound wrong before eventually drifting anyway. The SYNC button at this stage still can’t figure out musical phrases and the appropriate place to start at for you within the musical phrase(s). Where as if you actually knew how to do it manually you could place it as you pleased purely based on how it more intuitively sounds according to ear.

Ok, granted that most modern techno and house tracks are very DJ friendly with less complex musical phrase cycles for vastly easier seamless mixing.  But despite this there seems to be a fair few tracks that end up in my collection that are slightly more complex that use layered musical phrase cycles of different lengths on different instrumental elements within the same track. This is also the reason that when it does work out it might always sound different in landing at different points within the musical phrase cycle every time you mix it. 

Slightly more complex drum patterns with a syncopated drop beat more often than not confuses the auto SYNC feature, on such tracks it will never auto sync correctly no matter how many times you press the instant auto SYNC button. In such situations manually doing it is the only way to go and for all those that can't they end up limiting themselves to not being able to piece together a set that they had truly wanted to by just skipping it or not including it despite being a key element of the overall set in their minds eye. 

Its' for all the these reasons that I've covered in this and the previous post that I don't see the SYNC button as something that I would ever use unless I was working with simple 1 and 2 bar loops. It certainly won't spell the end of all the various perceived notions of DJ culture out there and a lot of the skill that is still involved.