If the cards are being changed out of view of the players you cannot be certain the decks have all the correct suit/ranks.
If they are using automatic shufflers you have no way to know for certain that they are shuffling the cards randomly. You could even be cheated in a brick-and-mortar casino. If you have any doubts then you should not play.
The problem is you have no way to know if you are being cheated. Some sketchier ones might well be rigged.
