Which one is better in terms of scrutiny by the customs?
Do they go by separate ways ?
Is it possible that the untracked packs get less attention from the customs? ( Maybe because tracked items are valuable so nicely taxable while untracked are not. )
Tracking has nothing to do with it. FCM international is perfectly trackable. The problem is there are only a few services that allow you to create the "LZ" type of labels. And I'm not going to tell you which ones they are.
But the "tracked" part of it is irrelevant. Customs scans the barcode and could give a shit whether the number is trackable. Where you might end up with extra scrutiny is the method you use.
For instance, Express is the MOST scrutinized. Priority is next most scrutinized. FCM and ParcelPost are the least scrutinized.
This hierarchy is established for some very simple reasons.
Drug dealers and Drug buyers tend to want to know where their shit is at all times.... and the tracking for Express tells you this. "Tracking" for FCM is really "Delivery Confirmation" and while is works as tracking... it isn't updated in a fashion that people associate with "tracking".
Drug buyers, and to a lesser extent drug dealers, are notoriously impatient. Hence Express and Priority are targeted as such.
Volume. The volume of FCM and Parcel post is likely many hundred times the volume of Priority and Express when it comes to international packages. It is much more efficient, and likely yields much better results, than trying to inspect any portion of FCM.
In summary, 'tracking' isn't a word that covers all the bases here, and it has nothing to do with amount of attention a package receives.