The reason of the 25 collections limitation is related to a Facebook limitation on the ad set level.
The creation AI module is setting 1 Facebook ad set per AdScale campaign, to optimize the performance and audience targeting optimization in Facebook.
Facebook limits the number of ads per ad set. Therefore, choosing more than 25 collections can bring the situation that some products and collections will not be included in your Facebook campaigns. To prevent that, we limit the number of collection you can choose per campaign.
In the case you would like to target more than 25 collections, we recommend to create an additional campaign.