... couls fail due to the epoxy breaking down ...
Ignition coils will fail due to excessive thermal cycling but epoxy break down alone is not the "root cause".
It is merely one of several possibilities to lead to the end result, known as the "general cause".
Internal break down, whether it be epoxy, windings or other numerous materials, are the direct result of excessive thermal cycling.
There is no wear-out factor in a coil itself other than thermal cycling and the damage it may cause.
There are many types or styles of ignition coils. Each with their own construction difficulties and possible failures.
Let's have a look at what the coil set out to do....
It is a timed event by means of sending voltage to fire a spark plug in order to create cylinder combustion.
Cylinder misfiring / lack of combustion
~ when a percentage of misfires rises within a coil, it's thermal content increases,
visual damage is often seen as cracks in the epoxy or housing.
What causes this ?
It may be due to a spark plug defect, a coil defect, defect in the boots seal or shorting associated within that cylinder.
When a connection to the spark plug opens or a plug becomes un-fireable, the voltage/heat within the coil will rise.
Worn, rounded spark plug electrodes aren't capable of efficiently focusing the high voltage.
As a result, it takes more voltage to fire worn plugs.
This "excessive thermal cycling" will create internal coil breakdown by means of further deterioration of the coil components.
The visual damage is often seen as cracks in the epoxy or housing.
We know the LS is known for leaking oil into the spark plug wells through it's inner boot well gasket. (not the outside gasket)
or even water and moisture from outside the engine compartment through the coil covers and coil boot housing seal.
This all (combined) leads to internal break down. Coil failures can also be the result of shorted windings.
... at least this is how I understand it but I've been known to be wrong before.
- - - - - EDIT:
@StinkinLS,
I wasn't suggesting that a leaking "oil filler tube seal" connected at the cover will result in failing coils.
It would only make a slight mess on the outside of your valve cover, at most causing oils to run near the hot exhaust manifold.
Just if you are doing your VCGs, it was something to look for/do to keep in check.