Homm5 Skill School

Comrades, many people talk about creating hero scouts, "roach" units, and so on, but at least a hero scout will survive until the first encounter with a normal enemy hero. The scout will either have to flee, resulting in a ransom payment at the tavern, or a new hero will have to be hired. Is such use of heroes rational in terms of money? If I'm wrong about the way scouting is used, please correct me. And another pressing issue: there's a lot of talk everywhere about tactics, especially how to counter "hit-and-run" tactics. But what if, by the end of the first week, Sinister arrives with 1-7 invisible units, casts spells, flees, and then his happy comrade arrives with the rest of his army and finishes the job? If possible, please answer for all factions.