Epub and .mobi are more or less zip files, like .cbz. The title page regularly is stored as an image inside, so it is easy for the server to pick it up and create a thumbnail. PDFs instead are commonly not simply image containers, so the server needs to proceed a totally different algorithm to get the title page for thumbnail creation. This it the main reason which makes PDFs so painfully slow for indexing purposes.

It does not matter whether it's a huge load of comics or books. The cause for this and similar issues is a leak on open file handles. It can be analyzed in the logs. In my case, ubooquite was running fine for approx. one month, and then I seem to have hit some sort of item limit. I have installed the server on my Synology DS916+ with latest DSM release 6.2.3 and JAVA8.

Yesterday suddenly icons of books and comics disappeared. This evening, when I added a couple of epub books, instead of adding them from one moment to the next one ALL comics vanished from the database. I even wasn't able to log on to the admin interface any longer, due to a JAVA exception "too many open files".

Since ubooquity is running with lots of other services on my DS, I guess it depends on those other services running at the same time at which point the OS (DSM is linux-driven) declines to give further file handles to ubooquity.

And at that point it is more or less random where the crash occurs, be it deleting entries from the database or corrupting the whole database at once.

It is too sad that the server hasn't been maintained/updated for mor than two years now, and it is even more sad that it is not open source - otherwise I'd been able to fix all the file handles and memory leaks on my own.