Asking for an idiom according to literal translation

Solution 1:

It's not clear to me exactly what OP wants to convey. If she's trying to point out that solving some particular problem is actually much harder than might be thought by others who don't understand exactly what will be involved, @Matt's "The devil's in the detail" is probably appropriate (it's often pluralised now, but I've always known it in the singular).

If on the other hand, OP is trying to convey that failure to conscientiously attend to all details will result in an inadequate solution, I suggest the proverb

If a job's worth doing, it's worth doing well.

Solution 2:

The best saying I can think of is

The devil's in the details

Which mean that the smallest parts of a problem are the most challenging.

The opposite of what you want, I think, is

Look after the pennies and the pounds will look after themselves

Meaning, if you take care of the little things the big things will fall into place.