Depends. If you're sure you're gonna run small groups of lights at different intensities, it might make sense to use more drivers.
If that's not the case, then why introduce more wires and plugs? The COB holders (the Ideal ones anyway) aren't rated for more than 250V, so it appears to me that Timber and etc. are putting as many COBs in a string as they can without breaking that limit.
I think the prevailing logic in matching COBs to drivers is based on 1) serial or parallel wiring, 2) the driver's voltage limit and maximum amperage, 3) the COB voltage requirement.
And, yes, the Meanwell "A" models have an internal adjustment, but the "B" drivers offer a wider output range than the "A"'s when you add a pot. I had a hard time wrapping my head around this whole subject, but it's really simple. The pot connects to the two Dimmer wires on the "B" models. Then you plug it in, then you twist the pot. In real life it's easy. But it's all gotta be right - you can solder the wires to the wrong leads on the pot, for instance.