According to consequentialism, an act (or a refusal to act in a particular way) should be judged by its consequences. But consequences can only follow from an act (or lack of action). So consequentialism is really founded on hope, with perhaps some justification in experience — if certain types of act (or inaction) are known to have certain consequences.
But even the simplest acts — those with a direct connection between their commission and the desired outcomes — can have unforeseen and unwanted consequences. Murder committed in the heat of the moment, but with the intention to kill, may land the murderer in prison or an execution chamber, neither of which outcome the murderer had in mind when he pulled the trigger or plunged the knife into his victim. Less dramatically, the outcome of a marriage proposal may — and usually does — lead not only to marital bliss (though perhaps not for as long as intended) but also to complications that hadn’t been contemplated (e.g., the raising of difficult children, financial stress, affairs, and other irritants large and small that strain the marriage).
Governance on the basis of consequentialism has proven time and time again to be foolish, when not treacherous. Social Security, for example, which was meant to be a boon to indigent old people has become a vast, economically draining, disincentivizing middle-to-lower-class welfare program. Social Security led to other vast and wasteful schemes, including Medicare and Medicaid and expansion through Obamacare, whose proponents made this fraudulent promise:
If you like the plan you have, you can keep it. If you like the doctor you have, you can keep your doctor, too. The only change you’ll see are falling costs as our reforms take hold.
Even successful wars — World War II, notably and uniquely in the American experience — have led to massive waste in lives and treasure. An annotated list of the ill-conceived and mis-conceived government ventures in the history of the United States would (and does) fill volumes.
I am not suggesting that persons refrain from taking action for fear that things won’t turn out as hoped for. (Government is entirely a different matter, and if most of it were abolished Americans would be far better off than they are.) What I am saying is that judging an action by its consequences can be done only after the fact, when all of the ramifications of the action have played out. Moreover, and crucially, similar actions can have wildly different consequences.
In sum: Consequentialism is an empty philosophical construct. “Good” consequences justify the actions that lead to them, but the actions have already been taken, and similar actions often have different consequences.