Obviously this is working for some people and not for others. Staying away from details about iOS 7 and Mavericks (which are both still under NDA), let me talk a little about a free open source project, libxml2.
Libxml2 is a library which parses plain text XML files, and ships with Mac OS X and iOS. It's used by OmniFocus to read your tasks, which are stored as plain text XML in your OmniFocus database.
Libxml2 version 2.9.0 was released as open source
last September, and introduced some default parser limits where there previously weren't any (other than available memory). These limits can be configured, of course—but if an app which was built against an earlier version of libxml2 were to find itself unexpectedly using 2.9 or later, it might very well accidently bump into these limits—particularly when asked to read large XML files.
That sort of change could explain why some people might have no trouble at all using an app, while other people find it stops working as soon as they try to load up a large dataset.
Hope this helps!