I think this is really a problem in the mid-game: I just got an hero offer with 3 ghosts deep in my homeland, no enemies and no 1st level ruins around, but I had another ghost and some scouts and higher-level ruins nearby, so I bought the hero, only to find out that the hero is entirely useless, despite having 23 undead lore. There is no way I can get the hero up to level 2 to search those ruins.
I understand that this feature shall prevent players from gambling an entire game on a lucky shot in the early game, but wouldnt it make sense to enforce it only in the first 10 turns or so? Or make it depend on the undead lore score, i.e. heroes can only enter ruins if they have above 50% undead lore for that ruin level (so a 1st level valkyrie cannot search any ruins all on her own - or maybe increase the percentage 60% to prevent lone 1st level assassins from searching too - for balance).
Or there should be an option to "train" heroes by letting them fight a unit in their stack or so.
