The updates come direct from Samsung on unlocked models. This means a few things...
Security updates are generally slower than carrier, not by much by Samsung tends to wait until it knows all carriers are using it without issue and then we get it, generally days later.
Major updates, ie to pie are quicker than on carrier. For instance the s9 upgrade to pie is now out for unlocked devices in a fair few countries. I would be surprised if carriers are pushing it out by mid January.
We generally get slightly longer term support for security updates. Carriers naturally lose interest in security updates as devices age, Samsung tend to support for 3 years and then sporadic beyond that.