The problem is the sheer fact that mathematically speaking, a "vanilla" un-epic, single scenario Death on Wings Encounter II is extremely unlikely not to be a one turn victory for the heroes if the win encounter I.
From a game design point of view, this is stupid.
I don't think the quest needs a boulder- the idea of adding an object to balance this quest in a standalone, certain hero count scenario seems unnecessary to me. The quest itself isn't really designed to be played (or balanced) in a vacuum- it's designed to be an Act 1 quest in a full campaign. In that context, if it's a 2 or 3 hero game, there are too many factors that affect overall campaign balance to say if this one encounter warrants adjustment- just like this quest is slanted toward the heroes, quests like "the ritual of shadows" are gimmes for the OL.
If you're playing it as a standalone encounter and feel you need to adjust the quest for your group's enjoyment, go for it.
Sorry, but this is wrong, or at least it runs counter to what the game is sold as.
Every quest with the exception of the Interlude and Finale is designed to be playable as a standard standalone, an epic qquest or part of a campaign, read your own rulebook if you do not believe me.
So I think it is perfectly legit to point out that it does not work for the most basic mode of play (single not epic quest) with any number of heroes but 4.
I shall not address the recurring suggestion for the webtrap, as, as I said, it is not about campaign or epic play.
Again, the problem is that it makes for more set up time than playtime in the aforementioned setup.
While not perfect about it, usually, quests do take the number of players into account, this quest does not in a very fundamental way. So, while not unique in this regard, this quest definitely is in need of fixing as much as Castle Daerion apparently was.
Yes, in a campaign, the strategy is of course more complex than just winning ONE quest, but in the default setting of a single quest, that indeed IS the strategy, all of it, so pointing to all those things is, frankly, a bit of a waste when talking about this quest as a standalone.