We would like that the deletion of content types and properties, that are part of the synchronization procedure during website initialization, should never abort to whole startup process if it fails (as it seems to be now). I'm refering to bullet 3.d. in this documentation https://world.episerver.com/documentation/Items/Developers-Guide/Episerver-CMS/9/Content/Synchronization/. This sounds more of non-critical part of the synchronization procedure. If correct, any failures in this part (e.g. due to some broken records in the database, infering db constraints) should preferrably only be ERROR-logged. 

Failure in this routine has made our production site unavailable twice after the application pool recycled during night. Exactly how the inconsistencies made it into the database is another question (but all removal of existing old content type definitions where made from the Admin UI...).

So if you can isolate this content type deletion part of the synchronization procedure, and if it's is not immediately critical to starting up/running the website, I think it makes sense to ignore any failures herein, making it more robust.

Please refer to this thread for the background: https://world.episerver.com/forum/developer-forum/-Episerver-75-CMS/Thread-Container/2018/2/failing-constraint-in-episerver-database---whole-site-unavailable/.