This issue has different results depending on how many times/much duration in exposure each screen and each ad unit has. If each screen has many times/much duration in exposure, putting in a dedicated ad-unique unit for each screen will maintain revenue and aloso make analysis easier. However, if you distribute different ad units to screens with low exposure, it may help analysis, but overall revenue will be low. This issue has been a bit more lenient on Android since Apple introduced its ATT(AppTrackingTransparency) policy.