If nightblood can cut on all three realms, presumably he can cut FROM all three realms and Szeth can transport himself to the cognitive realm along with nightblood if he has all 10 surges.
Also, you don't know if he will have the specialities of all classes of radiant as the surges are different depending on what you are. We don't know what effect this will have on the options he's got available, especially if he has "mastered" them all. But let's say he can fly and then soulcast the air around tlr into an aluminium iron maiden from a distance. Perhaps liquefy the ground he's on and encase him in rock, from a distance. Atium shadows? Lightweave a barrier tlr can't see through but szeth can and stay on the opposite side of it while he acts from a distance.
I think there is a lot of potential (on both sides) but there is far more known about allomamcy and feruchemy than surgebinding dynamics, hence why there seems to be a large number of people frothing over tlr being op rather than poor old szeth!