I have just tried it again, and for me, two old RnB/Disco-ish tracks sync as perfectly as they could. The trick is to re-press the sync button as soon as the tracks are running. Here is how I do it, maybe you can reproduce it when you know what’s different for me than for you.
I tried it with Djay on iOS (15.6.x), hooked up to my Traktor Audio 10, 2 turntables and a Numark DJ2Go2 Touch for some extra buttons.
I used the tracks “Kashif - I just gotta have you” and “B.B.&Q. Band - Imagination”. When these are just beatmatched, they will run out of sync immediately, because their speed varies quite significantly. With Sync enabled, they just sync.
I used the following settings:
DVS in relative mode
Settings->General->Sync Mode->Sync Type: BPM and Beats
Settings->General->Sync Mode->Turn off on pause, scratch or cue jump->disabled
Then play one track, cue up the second one and play it. Djay will disable the Sync function at that point. So just press the Sync button again to reengage, and voila, they stay in sync.
The fact that sync mode disables when you stop the record or cue the vinyl somewhat makes sense, because if you would force perfect sync when you do that, you would basically force the software to do something completely different than what the record does, making vinyl control useless. However, Djay seems to be able to cope with very light taps to slow down or speed up the record. This helps for the parts in songs with speed so irregular that Beat Sync can’t keep up.
So if anything, disabling the function “Turn off on pause, scratch or cue jump” does not do what the average user is expecting. Basically, it still turns off sync on scratch. I would expect it to be able to cope with syncing even if the downbeats are manually adjusted away fron another quite a bit, but the software seems to need them very close to another to work. If Algoriddim could improve on that, it would be much appreciated, but in principle, DVS/Beat Sync does work, at least for me.