The melting part is true.
Some Oil furnace companies state in the owners manual that the evaporator must be at least 6" above the furnace or airflow will be restricted. There are links on my pages to those companies.
I know a Tri-State (WI,IA & IL) Contractor that specializes in Oil furnace installations, he says never install a coil less than 6" above the Oil furnace if it has a large HT/EX near the top
or it will restrict airflow.
The amount of the back pressure varies with each installation situation. My brother's belt-drive is still only delivering half the airflow it needs for a mere 1.5-Ton A/C with a 2-Ton coil.
The problem is that the evaporator coils entry area is not large enough & the airflow is near the outer area, it hits the other areas & bounces back like a stream of water would causing turbulence & increased static. There is not enough air-space room between coil & the huge solid HT/EX to the entry area of the coil. Those factors do make a considerable difference.
If you install a filter grille or just a grille too close to the entry of a RA duct it will compromise airflow;
it needs a sufficiently deep box to realign to the duct entry area. Does that register with you?
Remember on some Oil furnaces the coil is very close to the HT/EX.
On a belt-drive blower motor that will work on heating without the coil, - few of them will move enough air due to the added back-pressure static, at their rated 0.50".
If the rest of the duct system is perfect & you have a multi-speed direct drive blower it may get by,
but at a higher static than it would if the coil provided more room for the air to get to the entry area of the coil.