Sure it worked wonders for the traffic. I have visited Boston (like downtown, not driving through) quite a few times and agree. The temporary works to carry traffic while construction was ongoing was nothing sort of amazing, but I can vividly recall as a 9 year old listening to my dad gripe about it as he was trying to navigate Route 1 to I-93.
I was initially referring to the excavation work through what is basically a landfill, pieces falling off tunnels, the mafia connections and the general consensus that only the local contractors made any money in the deal (big guys got screwed) among other things. All of that is aside from the fact that the Big Dig was supposed to be ~$3 billion (1990ish dollars) and ended up being closer to $10 billion, which you allude to.
I think you're right, if they were honest it would have never happened. But my grandparents/aunts/uncles living between Boston and the cape (but generally staying away from Boston) really felt like they were sold a bill of goods.