I believe the memory cap isn't even checked if you attach the debugger (as long as the device doesn't run out of memory completely). So checking without debugger might resolve some of the unpredictability. Nontheless it seems as if the DelegatingEffect might have some issues in certain cases. Given that the crash occurs outside the custom effects themselves (e.g. no out of memory when the effect tries to create the Buffers (as that isn't even being called).
It would however be great if we could create a sample with that issue that we can actually share for debugging purposes and perhaps submit to the Imaging SDK team to look into if we can't figure it out.