The first potential downside is that they simply make the VAST XML larger which can impact the download time for the user. Adding a few additional tracking pixels will not change the file size much, but adding lots of them (we have seen some VAST with 20 or more third party pixels added) can have an impact. The overall increase in size is still fairly minimal when compared to other media files (image, video, etc.), but small increases in file size can have an impact for users when using a mobile network. The impact of increased file size is less of a concern then the second downside.
The second potential downside is that a publisher may have a cap on the number of pixels it will fire for each video event (impression, video start, video quartiles, etc.). We have worked with publishers who capped the amount of pixels they will fire, such as 6 per event. These publishers fire the tracking pixels from their server rather than the client, so once this cap is hit they refuse to fire any more pixels even if more are listed. The publishers reasoning behind this decision is a desire to limit their bandwidth and server processing costs. If a publisher does not fire all of the pixels than the metrics an advertiser receives from the different third parties will not line up and can cause issues for a publisher receiving credit for all impressions recorded. From an advertiser’s perspective, they should make sure that their most important pixels are listed first to ensure they are always fired.
A publisher not firing all of the tracking pixels is not common, and most of the time they are fired from the client rather than the server so there is no cap. If you are a publisher who caps the number of tracking pixels you fire, then you should communicate this with your advertisers so they are aware of it. If you do not fire the tracking pixels from your server, then you should check with your video player to make sure there is no limit.
There are many things that can cause discrepancies between what an advertiser and what a publisher reports for videos displayed, so it is best practice for a publisher to communicate clearly with their advertisers how their ad serving environment works and to point out anything that is not standard and could cause inaccuracies in the reporting data.