One difference is that LU drivers can still see the monitors as the train leaves.
This may have crept in over time, but it’s only in the last few years that it’s become widespread. The Bakerloo and Piccadilly lines still don’t have this feature, and on all lines drivers can self-dispatch through an open cab door (at selected platforms) if the monitors are defective. When departing a platform LU drivers aren’t trained to continuously observe the monitors, especially on the non-ATC lines.
Interestingly, and as an aside, LU’s process doesn’t emphasise the “train safety check” in the same way as NR does. I would wager a fiver that at some point this will be a factor in an incident and will be changed.
I don’t think LU’s yellow lines were intended to be used as a yardstick for dispatch, and indeed offhand I don’t think they’re part of the current dispatch process, nor barely mentioned in their rule book. Although they are used as a yardstick by some, especially platform staff, again this would appear to be another case of mission creep.
A mainline driver has the monitors go blank once moving, meaning there's no real opportunity to stop the train quickly enough.
All of this highlights the rather rocky transition from the days of guards hanging out of the side of a departing train, as was the case on both LU and NR. It probably suited all that this transition tended to occur in the 80s when passenger numbers were at a low point. It seems we haven’t quite managed to find a seamless way of replicating the benefits this method brought - albeit with its own risks not least to the guard.