The Meridth effect was simple in theory, getting some sort of benefit wasn't that hard. Getting a lot of benefit was.
There is an awful lot going on.
The Mustang used a large radiator in area. The drag of a radiator is proportional to the speed of the air flowing through it.
IF you can reduce the speed of the air flowing through the radiator to 1/2 the normal airspeed the drag is 1/4 of the full speed drag.
The large, expanding air duct slowed the airspeed before the air went through the radiator matrix. This reduced the drag. The heated, expanded (higher speed?) air stream was then confined and the speed of the airstream increase further until it exited.
With the rather modest amount of heat warming the air stream (compared to actually burning fuel in the airstream) the shape/contours?change in cross section have to be carefully planed in order to get any real benefit. Too rapid an increase in cross section or too abrupt an angle increase in the duct the wall and could result in a disrupted airflow before the airstream even gets to the radiator creating excessive drag. If the airflow doesn't hit the radiator at a relatively even airspeed ( core of radiator isn't seeing much higher speed than the edges) then you don't get the advantage of the bigger radiator/slower speed through the matrix.
Getting the airflow speed
through the radiator matrix right so the air picks up the most heat with the least drag takes a bit of work too.
Speed too fast and you not only have higher drag, you may not get as high a heat transfer. Speed too slow and you may have not enough engine cooling going on and may not be able to get the airflow back up to the needed speed to generate any thrust ( getting the thrust to actually exceed the drag is the goal (it may not have been achieved) generating a few pounds of thrust (single digit HP) isn't much good if the drag is measured in tens of pounds (or tens of HP) or hundreds.
The Spitfire ducts weren't long enough and the Mosquito's weren't either.