What fierce attacks has bitcoin faced?
Sure, lots of people mock and criticize it, but no real action has been taken. The people criticizing it have just as much influence (or maybe even less) than the delusional and ignorant true believers.
To me, a fierce attack would be regulatory action taken by various world governments, but that hasn't happened. To the contrary, even countries that probably should care (e.g. China, with its capital controls, and bitcoin mining seemingly used to evade them) appear not to. If anything, that shows just how insignificant bitcoin is.
My vague impression (I don't follow this closely) of US regulators is that they have given bitcoin reasonable or even slightly favorable treatment.