Пандора
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Assemblers are only good for Archmages; for heroes with one or two spells, it's a waste of a slot, in my opinion. As for staves, only the one that gives a 20% bonus to resurrection fully justifies the two slots it occupies; all the others are only occasionally useful.
For non-Archmages, there are spellbooks that focus on a single army (they are, by the way, even more common). As for staves, I disagree; many of them are very worthwhile. The main thing is that the hero must be a mage.