From a design perspective, having your venture cards do something different contextually every time is excellent for keeping gameplay interesting, in theory. The issue is that the range of effect strength is divorced from the cost of the venture effects, so unless you design every room to be a 1 Mana "cost" effect, you risk wild imbalance in individual ventures, either on the side of too weak (which these dungeons err on the side of) or too strong. Either ends up boring or obnoxious. Back loading the power means that decks with only a few venture effects will find the Mechanic weak and boring, while decks with many venture cards will blast through the dungeons multiple times per game, averaging out their power level at the cost of any novelty or variety.
Restricting the dungeons to being separated by player instead of a loot race also makes them masturbatory fiddling with counters, in a place where you could have had both players be engaged.