Why do economists disagree so much?
Some areas of economics have more consensus and predictive power than others. Most economists would agree on the effects of trade barriers, could fairly accurately forecast the effects of a price change given a good demand estimate, would come to the same conclusion about the effects of allowing a merger between two large competing firms, know how asset prices will react to a change in interest rates, etc.

The effect of a fiscal stimulus is one of the most complicated questions in economics because you are asking about the effect of stimulating a system with millions of moving parts (people and firms) and many dimensions (consumption/saving, employment/work, trade, investment, innovation, ...). This is a long way from the simple, closed systems for which natural science is able to give sharp predictions. In fact, when you look at parts of the natural sciences that deal with similar levels of systemic complexity, the overall (lack of) predictive power looks similar to that in economics:


although principles of ecology are well understood, it is almost impossible to accurately predict what the final effect of, say, introducing a new species into an ecosystem will be
the theory of evolution is probably one of the greatest scientific theories of all time, but can only be used to make the vaguest predictions about the future
medics cannot accurately predict the onset of a wide range of diseases—only diagnose them ex post
models of future global temperature increases have wide error bars
weather forecasts more than one or two days into the future have wide error bars


The other thing to note is that fiscal stimulus is a highly politicised topic and economists often do a (frustratingly) poor job of keeping politics out of the debate. If you read the economic literature, you will find a story similar to that for your car crash: any outcome is possible but some look more likely, depending on the exact circumstances. Given this ambiguity, politicians of different persuasions have no difficulty finding an economist willing to ignore this nuance and take a politically expedient position (just as conservative politicians can always find scientists willing to play down anthropogenic global warming). But that is less a failing with economics than with economists.