Tifone,
High defense units could cause Hinnom some problems, especially since the Hinnom chariot is not as awesome as its stat might suggest. However, it is more an annoyance as opposed to a glaring weakness as Hinnom's sacreds have two attacks and Dawn Guards' doom sword provides an attack bonus.
The main weakness IMHO is that there are various pitfalls that a careless Hinnom player could fall into and shoot their own foot. In the hands of a good player Hinnom is difficult to counter. At least I have not found one yet (but I am working on it
)