From my experience, it is not so much if there are ads or not, but more of how they are being used. If you have an ad on the main screen that the player will be on for the majority of their time, then they are exposed to the ad more. If it's on a menu screen, that the player will always use, but not for a long period of time, then it is not as heavy of an exposure.
Another idea is if the player can get rid of the ad in some way, mainly navigation of the game. If the ad is on the main game screen, then when playing your game, they will always be exposed to it for as long as they play, which gives a kind of trapped sense to the player. If the ad is on a menu screen or something, the player can navigate away, and avoid seeing it. You are giving the player actions to prevent exposure to the ad, but exposure is the only way to prompt the action.
So exposure to ads is vital, but developers must also see to it that they avoid their strategy base to have accidental exposure , and particularly in major navigation areas of game screen.