I think that is over-simplified.
For starters Armstrong is brand new and the real proof of a player is not the dynamic debut but rather the long-term consistency. Let's see how Armstrong does against consistenly good opponents once they've had a chance to figure him out.
Secondly, Bula plays for the worst team in the comp 2 years running. The Knights arent great but as they showed against us and Titans they are gritty and prepared to work hard down the middle, which is key for the fullback support. Tigers on the other hand fall away in the middle as the match grinds on and instead throw Bula sideways football and hope he does something with it.
Thirdly, Bula's "cruise control" is already fast. You don't have to have your wheels spinning to be effective. Armstrong and Bula are not the same kind of athlete.
In this case I blame the output of the team rather than the lack of effort from the fullback.