hmm i think there are some weaknesses:
a) clicking up early leaves you vulnerable to castle age push with lots of mangonels, especially on arabia with lots of hills around your base, and often you have to build a castle at home to defemd that
b) the aggressive potential can be stopped by defensive structures in key locations (iirc viper defended a fast imp arb push vs vinchester?
the other day by just having 5 tcs)
edit: dont get me wrong i think it is still a very strong play on some maps and civ matchups.
im just trying to answer the question why it is not ubiquitous/overpowered etc