I can't agree with you (it seems you're contradicting yourself; read below, and you'll understand what I'm talking about). Practice shows the opposite (I'm talking about Kaspar). I don't understand what secondary heroes you're talking about when playing a rush strategy.
For example (I'm only talking about rushing) – on almost any map, with Kaspar, by the end of the second week, I have a level of 15-16, at a minimum, plus without any losses. With a successful build (this happens in 80% of cases), I have Fire Warriors and Skilled Machines – all of this allows me, with level 1 Spirits and 200 Skeleton Archers, to defeat neutral creatures of any level in almost 90% of cases without losses! Now, tell me, who can I not rush? I'm sure that with favorable conditions, you won't find a better rusher. This is my opinion based on practice.
P.S. I'm not a super specialist in Necromancers (I specialize in Gnomes), but I still believe that Kaspar is one of the best rushers in the game.