To me I think its is ok as long as there are no scenario search items in the scenario you want to play. I have all the expansions and just threw all the cards together, but then realized if I am playing Escape in the Truck it would be much harder to find gas and keys since there were so many more cards that I would be searching through.
That's true, but a lot of the extra cards you get with expansions and stuff balance out nicely in cases like that. For example, I believe Growing Hunger actually had more keys and even more gas for use as an explosive item, or to gas up the truck, and more dynamite. That, and of course, there are more of the card that allows you to specifically take a Scenario Search Item. I think it is "Just What I needed." So, in general it still basically balances out.