"substantially wrong" is a correct and usable phrase in written English.
It means that something is significantly, or fundamentally, incorrect or inaccurate. Example: Even though she thought she had followed the recipe correctly, the cake turned out to be substantially wrong due to a mistake in the measurements.
"It's been substantially wrong as far as choosing the candidate concerned," he said.
But as you can see in the graph above, the generic ballot average got both 2004 and 2012 substantially wrong, predicting a tie in 2004 and a Republican triumph in 2012.
But I did have a worry – which I took the precaution of tweeting on election day – that there could also be more specific reasons why the polls might be substantially wrong this time.
I discovered that the description of what happened contained in the Berkeley faculty letter turned out to be substantially wrong.
And even though the mayor has been open to discussing changes to the disciplinary system with the union, he hasn't said that there's anything substantially wrong with the system as it exists, only that it is old and "outdated".
Spinal syringes, needles, and other devices with connectors that will not also connect with Luer devices could substantially reduce wrong-route drug administration errors.
Algorithms are thus substantially driven towards wrong objectives.
The scenario deliberately contains three services rather than two services to emphasize prior findings that the expected inefficiency of use of OR time is increased substantially by selecting the wrong service to have its OR time released [ 20].
And in both, men with no distinguishing characteristic other than being born on the wrong date earned substantially less in civilian life than men who lucked out.
That led to a guilty finding under New York's insanity law, which holds the mentally ill responsible for crimes unless they are unable to distinguish right from wrong and are substantially unable to appreciate the consequences of their acts.
Responding to the government's initial proposal in July, Anderson had said that "such a board if properly constituted could bring advantages but the wrong decisions could substantially diminish the value that is offered by the current arrangements, particularly if there were any reluctance to share classified information with a larger and more varied group".
