It comes down to two things here. Both Galvin and Luai are run first halves. Their first instinct is to run rather than engage the line and create the gap for the second rower. They both like to skip across the field looking for lazy defenders or gaps for themselves.
The second is the backrower. Samuela does look to run that line ball but is still learning timing. He came through mainly playing front row so this is only his second full season of playing backrow exclusively. He's only 21 so he will get learn this as time goes on. He's still be great but he can elevate to a Olakau'atu with time. Seyfarth doesn't quite have the athleticism or explosiveness. He does occassionally run a good line, but usually he needs the half to make sure he is clean through without contact because he doesnt have the strength or athleticism to break tackles/the line effectively.
This is where Latu comes in, and personally why i've always been on the Latu over Galvin train. Galvin and Luai are runners, when they get the ball they look to skip across the line and try to find space for themselves. Whereas Latu likes to engage defenders by drawing them in, either creating the gap for the second rower to run the line ball or shifting the ball to take advantage of the numbers that he has created out wide. Perfect example is the Seyfarth try against Manly at Leichhardt last season. He gets the ball, squares up and engages his inside and outside man which creates a large gap in the line for Seyfarth to run through untouched.
In my opinion, thats why a halves combo of Latu and Luai has a greater chance of success. Greater balance of ball playing and ball running. Their success will be more based around what is put around them rather than their own abilities.